In experiments designed to investigate the biochemical basis for the diminution of the antitumor activity of 5-fluorouracil (FUra) by L-histidinol in CD8FI breast tumors, it was discovered that L-histidinol inhibits RNA and DNA synthesis in these tumors. This finding suggested the possibility that L-histidinol might have antiproliferative activity in the CD8FI breast tumor, and on that basis we evaluated different administration schedules of L-histidinol for antitumor activity in vivo. The present report describes a schedule of L-histidinol administration which yielded significant activity against spontaneous, autochthonous CD8FI breast tumors consistently at tolerable doses. To our knowledge, there are no previous reports of in vivo antitumor activity associated with the administration of L-histidinol as a single agent.