The effect of indomethacin treatment on hematopoiesis in mice was examined. Twenty-four hours following the onset of treatment there was 3-fold increase in bone marrow non-specific esterase positive cells (NEPC) as compared to the initial level. Two days later a two-fold increase in spleen and peritoneal NEPC was observed. In all the organs examined, NEPC returned to its basal level after cessation of treatment. The increase in number of NEPC was accompanied by an elevation in CSF production. The effect of indomethacin on peripheral blood cells was also determined. A significant decrease in hematocrit level was found following the treatment. In addition, it was further found that following the drug treatment there was a significant reduction of erythrocyte number and hemoglobin levels.