A team of California researchers has found that toxaphene, the nation's most heavily used insecticide, causes genetic abnormalities in bacteria that indicate it may also be a human carcinogen.

The California researchers reported in the latest issue of Science magazine that they produced mutagenic effects in salmonella bacteria with toxaphene. In May the same team reported that in a number of tests such mutagenic effects indicated a substance could cause cancer in humans.

The federally founded tests are expected to play an important role this fall when the Environmental Protection Agency decides whether to ban toxaphene. More than 100 million pounds of the pesticide are used annually in the United States, primarily on cotton, food crops and livestock.

EPA has been studying toxaphene since the National Cancer Institute conducted tests three years ago that showed the insecticide produced cancers in laboratory animals.

EPA officials said yesterday that NCI tests were not sufficient to determine whether toxaphene is a human carcinogen since they contained some procedural flaws. The officials said the California evidence would weight heavily in the agency's decision.

That ruling is likely to be made by November, said EPA Assistant Administrator Steven D. Jellinek. He said the EPA was anticipating a major battle from farm groups and pesticide producers over any attempt to pull the chemical compound off the market. Toxaphene accounts for nearly one-fifth of all pesticide used in the United States according to the agriculture industry.