Agriculture Department investigators delayed for nearly 1 1/2 months before they notified the Food and Drug Administration of a PCB contamination that has spread through food in at least six western states, federal officials said yesterday.

They said they have traced the contamination to a ruptured electrical transformer in a Billings, Mont., meat packing and animal feed firm.

Acting FDA commissioner Sherwin Gardner sent the six governors telegrams calling the contamination a "single incident" and saying that at this point consumer exposure to the contaminated food appears minimal.

However, federal officials said they were just beginning to trace the path of the PCB contamination. There were indications yesterday that the contamination, which apparently began in June and lasted until this month, may involve large quantities of food.

PCBs (polychlorinated biphenol), widely used in industrial machinery and electrical equipment, have been found to cause reproductive problems in humans and have caused liver cancer in laboratory animals. Use of the chemical compounds was banned by federal law in 1976.

An Idaho egg supplier said yesterday that it had to destroy millions of eggs and 300,000 chickens contaminated with PCBs in animal feed. Pepperidge Farm Co. said 75,000 of its frozen cakes made from contaminated eggs had been quarantined. A spokesman for the Butterfield Food Co. in Butterfield, Minn., said his firm had to dump 30,000 PCB-contaminated checkens.

Federal requirements limit PCB levels in chicken fat to no more than 3 parts per million. An Environmental Protection Agency official said yesterday that contaminated chickens had up to 167 parts per million of the chemical.

However, officials have said that the chance is low of consumers becoming seriously ill from PCB-contaminated food that has been sold.

Investigators for the federal Center for Disease Control in Atlanta were sent over the weekend to Idaho to test residents there to determine the extent of the contamination.

Kenneth Poggi, president of the Pierce Packing Co. of Billings, Mont., said in a statement that the PCBs apparently leaked from a transformer in a storage shed into a drain line. Federal officials estimated the leak began in June. Sludge from the line was combined with animal feed which in turn was shipped throughout the West.

FDA officials said yesterday that the first sampling of contaminated chickens and eggs was done by U.S. Agriculture Department investigators in Utah on July 7. But FDA spokesmen said their agency was not notified by the Agriculture Department until Aug. 20.

Donald Houston, administrator of the Agriculture Department's division of food safety and quality service, said yesterday that two weeks is the normal time for a sample to be processed through the division's laboratory and then reported.

However, Houston said that the PCB-contaminated sample taken in Utah took 3 1/2 weeks to reach the department's western regional office in Alameda, Calif. That office eventually notified FDA, which began its own investigation.

"There was a delay and I can't explain it at this time," said Houston. "We're working on it right now."

During the time lag, investigators said, animal feed from the Billins firm was shipped through several Montana cities and distributed across the West.

The states notified by the FDA of the problem yesterday were Minnesota, Montana, Idaho, Washington, North Dakota and Utah.