We examined predictors of organochlorine concentrations in serum specimens from women who were pregnant in the 1960s and participated in the Child Health and Development Study in the San Francisco Bay Area of California. That study enrolled pregnant women at the Kaiser-Permanente Medical Facilities, conducted interviews, and drew blood specimens; these specimens were centrifuged and the resulting serum specimens were frozen and placed in long-term storage. For the current investigation, organochlorines were measured by dual-column GC-electron capture detection in specimens collected in 1963-1967 from 399 pregnant women during the second and third trimesters. Using multiple linear regression models adjusted for serum lipids, we evaluated factors predicting concentrations of 11 polychlorinated biphenyl (PCB) congeners, their sum, and several pesticides and metabolites. Variables evaluated were age, race, place of birth, date of blood draw, body mass index, occupation, past residence on a farm, parity, and duration of pregnancy at blood draw. Concentrations of highly chlorinated PCBs and the sum of the PCBs increased with age. Concentrations of certain PCB congeners, as well as the sum, were significantly higher among nonwhites and increased with calendar date of blood draw. (italic)p,p(/italic) -DDT and (italic)p,p(/italic) -DDE concentrations were about 50% higher for nonwhites compared with whites and for those born in California or the southeastern United States versus elsewhere in the United States. Higher body mass index was associated with lower concentrations of several PCBs and (italic)p,p(/italic) -DDE but with higher heptachlor epoxide and DDT levels. The increase in use of PCBs during the 1960s is apparently detectable as increasing concentrations in maternal sera between 1963 and 1967. Marked racial and regional differences in serum pesticide levels were likely caused by geographic variation in previous agricultural and vector-control uses. The relationship to body mass index appears to be complex.