The exposure of human lymphoid leukemia Molt 4B cells to sodium 1-monolinolenin (SML) which was isolated from the leaves of Italian ryegrass (Lolium multiflorum Lam) and identified by Mass, and (1)H- and (13)C-NMR, led to both growth inhibition and induction of programmed cell death (apoptosis). Morphological change showing apoptotic bodies was observed in the Molt 4B cells treated with SML. The fragmentation by SML of DNA to oligonucleosomal-sized fragments, that is a characteristic of apoptosis, was observed to be both concentration- and time-dependent. These findings suggest that growth inhibition by SML of Molt 4B cells results from the induction of apoptosis in the cells.