Of all members of the medical profession, doctors would appear to be the most unlikely to join unions. Indeed, a majority of the nation’s nearly 800,000 doctors are prohibited by federal antitrust laws from collective bargaining because they are considered to be self-employed.

But the rise of managed care, more salaried doctors and unremitting cost constraints at county health departments are adding more doctors to union ranks, especially in L.A. County.

Two years ago, the county’s 800 doctors became the largest group of physicians to unionize in decades when they joined the Union of American Physicians and Dentists, an Oakland-based group with only 6,000 members. It claims to be the largest doctors’ union in the country.

It was able to negotiate a new contract for the doctors that includes such advances as on-call pay, standardized continuing education benefits and a salary hike of 8.75 to nearly 20 percent.

But the doctors are finding that unionizing has been far from a panacea, especially when the county made them ineligible for its popular “Megaflex” benefits plan, a sizable cash bonus that can be used to supplement benefits or pay. County doctors say that at an average salary of $110,000 annually they are below physicians in the private sector.

And even with the raises, the union concedes the majority of its members would be losing money under the new contract.

The county says it took the action on Megaflex because only non-union employees are eligible for the plan and doctors should have known that. But union officials say the move is an effort to break the union, and the county has angered many doctors who now blame the union for losing the benefit.

“I think they are angry and upset and feeling helpless,” said Dr. Belinda Wu, a pediatrician at the Clinic H. Claude Hudson Comprehensive Health Center in Los Angeles who sits on the union’s board.

Wu said she tries to remind her colleagues that the county had also trimmed the Megaflex plan in a prior cutback in 1995 before there was a union.

But the union has filed an unfair labor practice charge, plans to file a lawsuit against the county and is hoping for passage of a state bill that would force the county to binding arbitration.

In the meantime, it is organizing doctors at a small public health authority in the San Gabriel Valley.