Reflecting a national trend toward unionization of medical professionals, the District of Columbia Board of Labor Relations yesterday certified a national labor organization as the bargaining representative for 193 junior doctors at D.C. General Hospital.

Initially, a local unit of the Physicians National Housestaff Association will bargain for working conditions for all physician interns, residents and fellows at the city facility in Southeast Washington. Senior medical staff members are not involved.

Salaries cannot now be negotiated, but the City Council's recent enactment of a new municipal personnel law could make that possible in 1980.

The housestaff association is a national union that represents about 12,000 doctors at 50 publicly owned hospitals across the nation. Its certification at D.C. General was based on a petition from affected doctors.

Gail Britton, staff organizer for the union, said the only other public hospital with such a bargaining unit in the Washington area is Prince George's General Hospital in Cheverly, where a 75-member unit is currently negotiating its first contract with management.

Britton said there is only one other known bargaining agent for hospitalemployed doctors in Washington, at Children's Hospital, which is operated by a private charitable organization. The bargaining agent is an unaffiliated local organization.

The best known physicians' union in the area represents staff members of the Group Health Assocation, a 109,000-member organization that operates a prepaid health plan with clinics but no hospitals.

Last April, the GHA union conducted the nation's first major strike by doctors.It lasted 11 days and resulted in a contract that will bring average salaries to $60,000 by next year.

Britton said the D.C. General junior doctors are in a far different economic condition, working long hours for low wages. Although no bargaining demands have been prepared, she said the union will seek improved working and patient-care conditions.

Officials of D.C. General could not be reached for comment.