Private-equity investors are combining the nation's largest security-guard services into a single giant company. In the biggest merger, announced this morning, AlliedBarton Security Services, the Conshohocken company that employs 60,000 contract security guards for companies and landords nationwide, says it plans to merge with Universal Services of America , a Santa Ana, Calif. rival with 80,000 workers (including janitors as well as security guards), later this year.

The combined AlliedUniversal will have sales of $4.5 billion a year. It will control at least 15 percent of the U.S. security guard market, according to data for industry leaders that AlliedBarton shared with investors last year.

Universal boss Steve Jones will serve as CEO of the combined companies. AlliedBarton boss William C. "Bill" Whitmore Jr., a former police officer who headed AlliedBarton predecessor SpectraGuard, will chair the AlliedUniversal board. Whitmore "is really a character in this industry. He's a very important guy to know," Frederic Lemoine, chairman of France's Wendel private-equity group, told investors after buying AlliedBarton last summer.

Universal and AlliedBarton didn't announce a purchase price and are apparently trying to structure the deal as a tax-free merger of equals, though Universal, which has nearly doubled in size through mergers in the past nine months, is larger than AlliedBarton.

When guard companies merge, cost cuts "are quite quick to be materialized, because it's more or less a question of merging offices and sales forces and administrative support functions," Lemoine told investors in a conference call last month. The companies say they will keep both headquarters offices open, adding that more details might be announced by managers after the deal closes later this year.

"This industry has a fantastic potential for consolidation," with dozens of merger targets, Lemoine added in the investor meeting. He said AlliedBarton had been growing 5 percent a year since 2011, even without acquisitions, "fueled by an ever-increasing need for more security services in North America," "cheap" prices for regional security-guard firms, and highly profitable cash flow: "These are the main reasons why we are invested into this excellent business."

Lemoine told investors the guard industry, dominated by former police and soldiers, is "very fragmented," showing "different degrees of unionization" and "no harmonization." (corrected) Labor accounts for about two-thirds of the company's costs.

Labor organizers say they are ready to press the company and its global investor-owners to share its growing revenues with the workers it relies on. "We are seeing huge companies grow to the scale that manufacturing companies reached, and consolidating on a global scale, in the service sector-- security, janitorial, food services," said Gabe Morgan, director of Service Employees International Union Local 32 BJ, which represents 165,000  guards and building cleaners and engineers in New York, Philadelphia, and other East Coast markets.

Morgan told me a test for AlliedUniversal will come this fall, when contracts for 2,500 Philadelphia security guards, including AlliedBarton staff, come up for renewal. SEIU just concluded a Washington, D.C. contract boosting guard wages to start at $17.45 an hour, a big step up from the $10.45 plus healthcare the lowest-paid Philadelphia SEIU guards won in their first contract in 2012. SEIU advocates a national minimum wage of at least $15. "No matter what the name is on their shirt, we find workers want to organize," Morgan added. "This is where the work is."

The merger comes less than a year after Wendel bought around 95% of AlliedBarton (itself a product of earlier mergers) from U.S. investor Blackstone Group for a price eventually marked at $688 million (more than 11 times EBITDA cash flow).

At that time, AlliedBarton said it employed 55,000 guards and other workers, including around 6,000 in the Philadelphia area, working at more than 3,000 companies, including 200 of the Fortune 500. AlliedBarton's 2014 sales totalled $2.2 billion.

Also last summer, in July, New York-based investment firm Warburg Pincus and Partners Group said it made "a major investment" in Universal, which at the time employed 44,000 guards.

Later that month, Universal's Protection Service bought Guardsmark Group, a Memphis guard service with more than 15,000 guards.

And in October, Universal Protection Service agreed to pay $131 million to purchase ABM Security Business, another large security-guard provider, from ABM Corp.