Through our antiviral screening work with medicinal plants of the Pacific areas, we have found a narcissus alkaloid (pretazettine) with therapeutic activity against Rauscher (myelo or erythro) leukemia. Different from standard anticancer drugs, the alkaloid could be administered for long term without loss of body weight or other side effects. We now report the definite activity on spontaneous AKR (lymphocytic) leukemia used as another good model of human acute leukemia. The alkaloid has only a moderate activity against several transplantable (nonviral) tumors in mice, but it produces some additive effects when combined with several standard anticancer drugs. Although the alkaloid is inhibitory to viral reverse transcriptase, it specifically inhibits cellular protein synthesis in leukemic blood cells in mice.