Cytotoxicity of 4 Aroclors (1016, 1242, 1254 and 1260) was compared in Chinese hamster ovary (CHO-K1) cells in Ham's F-12 medium. When parameters of toxicity were cell numbers or tissue protein, 50% lethality occurred at Aroclor concentrations between 30 and 45 ppm. An in vitro clonal assay with CHO-K1 cells was a sensitive indicator of cytotoxicity of the polychlorinated biphenyls (PCBs). From EC50 values (concentration that allowed 50% survival of formed colonies), cytotoxicity was lower with Aroclor 1016 (32 ppm) and higher with Aroclors 1254 (27 ppm) and 1260 (28 ppm). In cells exposed 24 h to a marginally cytotoxic dose (20 ppm) of each Aroclor, phospholipid (PL) thin-layer chromatography (TLC) showed an increase in phosphatidylcholine (PC) and a decrease in phosphatidylethanolamine (PE) and diphosphatidylglycerol (DPG). Neutral lipid (NL) TLC of cells given Aroclors 1242, 1254 or 1260 showed a 3-4-fold increase in triglyceride (TG) and a similar reduction in cholesteryl esters (CE); in contrast to Aroclor 1016 which produced no change in TG and a smaller (2-fold) reduction in CE. Cholesterol and free fatty acid fractions were unaffected by any of the Aroclors. The TG:PL ratio remained unchanged in cells given Aroclor 1016, but increased 3-4-fold with Aroclors 1242, 1254, or 1260. Compared to total values in the untreated controls, CHO-K1 cells contained less neutral lipid and more phospholipid only with Aroclor 1016. These results support the concept that differences in the behavior of Aroclor 1016 are related to its PCB composition. Changes in membrane PL and NL components, observed at marginally cytotoxic levels of each Aroclor, provided further evidence that the PCBs may affect membrane integrity and associated metabolic functions.