The antitumor activity and hematopoietic toxicity of two busulfan analogs were evaluated in comparison with those of busulfan. Although a program of five daily ip treatments with busulfan was not effective in treating sarcoma 180-bearing mice, a fluorine-containing busulfan analog, 1,4-butanediol di-2,2,2-trifluoroethanesulfonate (BFS), and a water-soluble analog, 1,4-butanediol diisethionate (BIT), were significantly effective when given on the same schedule. Busulfan did not appreciably prolong the life span of either P388- or Meth A-bearing mice, whereas BFS and BIT produced significant increases in the life span. It is worth noting that both the analogs were definitely less toxic to the host mice than busulfan. All the drugs examined exhibited suppressive effects on the counts of total WBCs, neutrophils, and lymphocytes. Relative toxicity toward neutrophils versus lymphocytes was increased significantly in the BFS and BIT treatments compared with busulfan treatment. It seems that the toxicity of busulfan in host mice might be due to unidentified side effects other than bone marrow suppression. These results suggest that BFS and BIT could be improved substitutes for busulfan.