Nearly half of all new chemicals are submitted to the Environmental Protection Agency for review without information on their toxicity, and only 17 percent come with information about their potential for causing cancer, birth defects or genetic mutations, according to a new Office of Technology Assessment study.

The study, released yesterday at a House reauthorization hearing for the Toxic Substances Control Act, said the absence of such data "complicates" the EPA's efforts to decide whether a new chemical poses an unreasonable risk.

But it added that the lack of the data wasn't surprising, because the act doesn't require it.

The report by the bipartisan congressional study office was among the opening shots of a fight to tighten the provisions of the act, enacted in 1976 after a public uproar over polychlorinated biphenyls (PCBs) and other chemicals that were in common use long before their potential hazards were known. The law gives the EPA broad powers to regulate dangerous chemicals before they enter the marketplace.

In testimony before a House Energy and Commerce subcommittee, EPA officials conceded the law got off to a slow start, but they contended it is now gaining momentum and urged the panel to approve a simple two-year reauthorization without major revisions.

"I feel that now priorities and procedures are largely in place, and the program is moving ahead to accomplish the things which Congress intended," said Don R. Clay, acting assistant administrator for pesticides and toxic substances.

Industry witnesses also urged no revision of the law.

But an environmental lawyer, flanked yesterday by an electric utility official and an executive of a commercial waste disposal firm, testified that the law was suffering from inertia under the Reagan administration's "voluntary" approach to data-gathering and contained serious loopholes that may have compromised public health.

Jacqueline Warren, counsel for the Natural Resources Defense Council, said the law has failed "to put an end to the 'use first, test later' approach to chemicals manufacture that had given us PCBs, vinyl chloride, PBBs and a litany of other notorious chemicals."

"Six and one half years later, it is clear that we are still allowing most new chemicals to enter commerce with little or no toxicity testing," she said.

Of particular concern to Warren, and to the waste disposal and utility officials with whom she shared the witness stand, was the law's provision for PCBs.

PCBs, cancer-causing chemicals used in electrical equipment, are the only toxic substances Congress specifically ordered the EPA to address through the toxic substances act. The disposal of other hazardous wastes is governed by the Resource Conservation and Recovery Act, which has stringent record-keeping rules and requires disposal or incineration in licensed facilities.

Environmentalists contend that the EPA, under the act, is allowing the disposal of PCBs at unlicensed facilities, without records being kept. Warren called it a "regulatory loophole through which possibly huge volumes of PCBs are disappearing across the country."