The exposure of human lymphoid leukemia Molt 4B cells to honokiol led to both growth inhibition and the induction of apoptosis. Morphological change showing apoptotic bodies was observed in the cells treated with honokiol. The fragmentation by honokiol of DNA to oligonucleosomal-sized fragments that are characteristics of apoptosis was observed to be concentration- and time-dependent. These findings suggest that growth inhibition by honokiol of Molt 4B cells results from the induction of apoptosis in the cells.