Federal investigators who took soil samples from the back yards of 12 homes near Harbor Gateway in September were looking for contaminants that might have spread from the nearby Del Amo Pits, a proposed Superfund site.

But they were surprised to find that, while none of the soil samples contained unsafe levels of the toxic chemicals they were searching for, two of the samples contained unusually high levels of the banned insecticide DDT.

Investigators, who announced their findings recently, were in the neighborhood to check whether any heavy metals from a former rubber manufacturing plant nearby had found their way into residents' yards.

The former plant, a 280-acre site known as Del Amo Pits, produced such byproducts as benzo(a)pyrene, a carcinogen, said Tom Dunkelman, U. S. Environmental Protection Agency project manager. He said the agency is at a loss to explain the source of the DDT.

Although EPA officials do not believe the DDT concentrations are high enough to cause illness, they intend to conduct additional tests next month at the two homes where DDT was found. If the next round of more intensive tests reveal high levels of the pesticide, federal officials might replace the soil.

"These are not levels that would cause anyone to become sick over a short-term exposure," Dunkelman said. "The way in which EPA evaluates risk is very conservative and puts safety first. The risk numbers assume that people ingest a certain amount of soil every day for years."

Once a widely used pesticide, DDT has been banned in this country since 1972. People exposed to high doses of the contaminant for short periods typically suffer tremors, rashes and irritation of the eyes, nose and throat. DDT also has been found to cause cancer in laboratory animals.

Typically, federal officials recommend taking precautions when the concentration of DDT exceeds 100 parts per million, although recent changes in how authorities assess human risks will probably lead to more stringent standards in the next few weeks.

Samples from one back yard yielded DDT at a concentration of 32 parts per million, while the other contained DDT at 110 parts per million, leading the agency to advise the residents to wash their hands after working in the yard. Tests at the other 10 homes detected only traces of DDT, no more than 5 parts per million.

The two homes with soil containing substantially higher levels of DDT are next to each other on West 204th Street, between Normandie and New Hampshire avenues in an unincorporated area.