Major drug companies that sell brand-name drugs made in other firms' plants fail to maintain their claimed control over manufacturing by putting their own "man in the plant," a congressional subcommittee said yesterday.

The finding is the latest shot in a running battle over the comparative quality of brand-name presciption drugs and less expensive generic, or unbranded drugs.

Many major drug companies maintain that their brand-name products are superior to less expensive, unbranded products. Advocates of wide use of generic drugs say this isn't so in most cases, and point out that many of the brand-name drugs are made for the bigger firms by generic manufactureres.

The House Commerce Committee's sub-committee on oversight and investigations maintained yesterday that:

The so-called "man in the plant"-the bigger firms' representative in the plant that really makes the drugs-performs only clerical duties, rather than any that affect manufacturing processes. The drug companies insist that they monitor these drugs' quality in several ways.

The Food and Drug Administration has found that generic drugs are generally "therapeutically equivalent"-no better or worse than brand-name drugs, on the average-and brand-name firms have given the sub-committee no 'substantiating evidence . . . of higher quality."

The FDA and the Department of Health, Education and Welfare, of which it is a part, do not require brand-name firms selling drugs made in other plants to reveal the manufacturing site on the label.

Rep. John E. Moss (D-Calif.), subcommittee chairman and a crusader on this and many other health issues, will retire at the end of this term. He and his subcommittee urged Congress to "continually monitor" FDA to make it stop all misleading advertising and labeling. They urged Congress to broaden investigation of drug advertising, marketing and pricing.

The FDA has maintained that a new drug regulations revision bill-to be considered in Congress next year, following a start this year-will solve the labeling problem. HEW, meanshile, has proposed a new regulation to modify labeling.

But the FDA already has power to require all manufacturing sites to be revealed to the public and should do so, the subcommittee said.