L-Histidinol, a structural analogue of the essential amino acid L-histidine, enhances the toxicity of a variety of anticancer drugs for many tumor cells of animal origin. In this study, the effects of L-histidinol on the proliferation and anticancer drug susceptibility of two human tumor cell lines of lymphoid origin, Daudi and MOLT 4, have been examined. L-Histidinol increased the inherent capacity of six different antineoplastic agents to kill these human tumor cells, in a dose- and time-dependent manner, in spite of the observation that it slowed cell cycle progression in both lines.