Polylactic acid (PLA) microspheres containing bleomycin were prepared by a solventevaporation process. The release rate of bleomycin from PLA microspheres was very small. The release profiles of the drug from microspheres prepared by using L-and DL-isomers and PLA of different molecular weights did not differ significantly for up to 120 h. The release rate of the drug could be greatly enhanced by incorporating fatty acid esters as additives in the microspheres. It was found that the release rate of the drug could be controlled by using different additives and by adjusting the amount of the additive. Scanning electron microscopic observations suggested that the additive was uniformly distributed in the microspheres as small droplets. The enhanced release rate of the drug in the presence of the additives can be attributed to the highly porous structure of the PLA matrix through which the drug is released by diffusion.