4-Methylthio-2-oxobutanoic acid is the direct precursor of methional, which is a potent inducer of apoptosis in a BAF3 murine lymphoid cell line which is interleukin-3 (IL3)-dependent. Cultures treated for 8 h with methional in the presence of IL3 show extensive DNA double-strand breaks on flow cytometric analysis, increases in DNA fragmentation as measured by the amount of non-sedimentable DNA present in the 30,000 g supernatant of cell lysates and the typical laddering pattern of multiples of 180 bp seen upon agarose gel electrophoresis. No such features of apoptosis were found in cells treated with 4-methylthio 2-oxobutanoic acid or propanal, suggesting that the simultaneous presence of the methylthio group on the propanal moiety is essential for apoptosis to take place. Methional is further metabolized in cells by two reactions: oxidation via aldehyde dehydrogenase to (methylthio)propionic acid or beta-hydroxylation to malondialdehyde. The formation of malondialdehyde from methional in vitro by chemical hydroxylation under the conditions of the Fenton reaction provides a mechanism for the beta-hydroxylation which takes place in vivo. During apoptosis induced by IL3 deprivation, the ratio of 2,4-DNPH MDA to 2,4-DNPH methional is 0.94 in cells in IL3- medium compared with 0.54 in cells in IL3+ medium. These results support a role of cellular methional and malondialdehyde in apoptosis.