A family of polyunsaturated fatty acids is shown to be abundant in utero during pregnancy. Members of this family are able to induce cleavage of poly(ADP)ribose polymerase, and ultimately to induce apoptosis, in T-antigen-transformed Jurkat T cells. Free radical scavengers, including phenol and benzyl alcohol, block the apoptosis-inducing properties of these polyunsaturated fatty acids; this is consistent with a lipid peroxidation mechanism involving formation of hydroperoxy fatty acids.