Ricin D, one of the two isolectins from small caster beans showed stronger cytotoxicity than another one, ricin E, based on the inhibition of colony formation and the inhibition of protein synthesis. Both ricin D and ricin E induced cell lysis to different extents in each cell line tested, albeit ricin E was slightly less effective than ricin D. DNA fragmentation, a characteristic feature of apoptosis, was also induced by ricin D and ricin E in Vero cells. Scatchard plot analysis showed that ricin D binds to cells with higher affinity than ricin E, while the number of binding sites per cell was not much different, suggesting that the differences in the cytotoxicity between ricin D and ricin E is mainly due to their differential binding affinity to cells. In Vero cells, the cytolytic activities of ricin D and ricin E were inhibited by brefeldin A (BFA), which is known to effect the Golgi apparatus, but not significant effect of BFA was observed in a BFA-resistant cell line, MDCK cells. These results suggest that the Golgi apparatus may be involved in ricin-induced cell lysis.