To examine the possibility of utilizing albumin microspheres containing 5-fluorouracil (5-FU) as a drug carrier, the antitumor activity of albumin microsphere-entrapped 5-FU against Ehrlich ascites carcinoma and solid tumor in mice was studied. After intraperitoneal injection of microsphere-entrapped 5-FU into Ehrlich ascites-bearing mice, the 5-FU level in the ascites was very high compared with that after injection of the free drug. The suppressive effect of microsphere-entrapped 5-FU on tumor growth in ascites-bearing mice was significantly higher than that of the free drug. After multiple-shot administration of microsphere-entrapped 5-FU to ascites-bearing mice, the increase in life-span was over 50% compared with a control. The therapeutic effect of microsphere-entrapped 5-FU on Ehrlich solid tumor after intratumoral injection was also studied. 5-FU level in the tumors of mice was significantly higher after intratumoral injection of microsphere-entrapped 5-FU than when the free drug was administered. The injection of microsphere-entrapped 5 FU, which was slowly released from the albumin microspheres in the solid tumor, caused a higher suppression of tumor growth at the inoculation site than administration of the free drug at the same dose. These results suggest that albumin microspheres containing 5-FU may represent an effective system for drug delivery, with prolonged action.