In the present study, the preparation and characterization of bovine serum albumin (BSA) microspheres and the evaluation of the in vitro cytotoxicity of these microspheres on acute promyelocytic leukaemia (HL-60) cells were described. Mitoxantrone (MTZ)-incorporated microspheres were evaluated for particle size, drug loading, release characteristics and surface morphology. The biological effect of MTZ released from BSA microspheres was determined on an in vitro cultured HL-60 cell line, showing that, after encapsulation, MTZ still retains cytotoxic activity. For this purpose, methyl-thiazol-tetrazolium (MTT) assay was used to evaluate the in vitro cytotoxicity of MTZ-loaded microspheres. Particle size of BSA microspheres was determined between 17.61-20.38 μm and they were smooth and spherical in shape. Encapsulation efficiency of the drug-loaded microspheres was between 22.26-60.50%. For MTZ-containing microspheres, the cell death ratios were greater than 80% for all formulations. This study demonstrate that BSA microspheres were well suited for the controlled release of MTZ and were promising for anti-cancer chemotherapy. © 2004 Taylor & Francis Ltd.