Release profile of 5-fluorouracil (5-FU) from O/W emulsion and bone marrow toxicity of 5-FU to mice were compared with those of an aqueous solution of 5-FU. 5-FU emulsion had lower bone marrow toxicity and a slower drug release rate than the 5-FU aqueous solution which exhibited high bone marrow toxicity and fast drug release rate. A correlation between the rate of drug release and bone marrow toxicity was suggested.