Parinaric acid, a naturally occurring 18-carbon fatty acid containing 4 conjugated double bonds, is toxic to human monocytic leukemia cells at concentrations of 5 microM or less. Conditioning of the medium reduces the cytotoxic effect, suggesting that parinaric acid and not a metabolite is the active agent. The mechanism of parinaric acid toxicity appears to involve lipid peroxidation because the toxic action can be blocked by the addition of butylated hydroxytoluene. When U-937 cells are differentiated to the monocytic form, they become resistant to as much as 30 microM parinaric acid. This difference in sensitivity may be explained in part by the fact that the undifferentiated cells take up 3 to 4 times more parinaric acid. Concentrations of parinaric acid less than 5 microM are also toxic to human THP-1 monocytic leukemia, HL-60 human promyelocytic leukemia, and Y-79 human retinoblastoma cells. Measurements of protein synthesis indicate that differentiated U-937 cells, confluent cultures of human fibroblasts, bovine aortic endothelial cells, and CaCo-2 colonic mucosal cells are much less sensitive to parinaric acid than the malignant cell lines tested, suggesting that the cytotoxic action may be selective for rapidly growing malignant tumors. Thus, parinaric acid may be the prototype of a new class of lipid chemotherapeutic agents that contain a conjugated system of double bonds and act by sensitizing tumor cells to peroxidation.