Antitumor activities of harringtonine (HA) and homoharringtonine (HO) both belong to cephalotaxus alkaloids were compared with those of vinca alkaloids, vincristine (VCR) and vinblastine (VLB). HA and HO had significant activities against P388 leukemia, L1210 leukemia and B16 melanoma by intraperitoneal injection comparable to VCR and VLB. The therapeutic index (LD₁₀/ED₅₀) of HA and HO in B16 melanoma system was 1.90 and 2.31, while those of VCR and VLB were 1.38 and < 1.00, respectively. The oral administration of these cephalotaxus alkaloids evoked a significant antitumor activity but that of vinca alkaloids induced no activity against P338 and B16 melanoma. The efficacy of HO on various tumor systems was equal or greater than that of HA when comparison was made between these two cephalotaxus alkaloids. In contrast to VCR and VLB, HA and HO demonsrated moderate activity against P388/VCR, a subline of P388 resistant to vincristine. The intraperitoneal injection of VCR or VLB produced an increase in the mitotic index of P388 cells as compared with the control values, but HA or HO decreased it. HA and HO, cephalotaxus alkaloids are a new class of active compounds of plant origin which may differ from the vinca alkaloids in the mechanism of antitumor activity.