A phase II study of the use of colony stimulating factor derived from human urine (CSF-HU) was performed after bone marrow transplantation (BMT). Steady and rapid recovery of leukocyte and granulocyte numbers was observed. In most patients who received CSF-HU from day 1, leukocyte numbers started to increase on day 6 and monocytes and granulocytes on day 11. Significant differences in the days to recovery of leukocytes over 1 x 10(9)/l and of granulocytes over 0.5 x 10(9)/l were observed in comparison with non-randomized control patients. In some patients in whom recovery of leukocytes was delayed, CSF also seemed to be effective in increasing leukocyte numbers from 8 days after the start of administration. There was no significant difference in the rate of relapse of leukemia between the two groups. CSF-HU seems very promising as a treatment of patients after BMT by shortening the period of leukopenia or granulocytopenia.