A comparison of the cytotoxic effectiveness of adriamycin incorporated into ion exchange microspheres with conventional chemotherapeutic use of adriamycin was carried out in a rat tumour model. Drug microspheres were targeted to the tumours by embolisation into the arterial supply of the hind limb bearing the tumour. Microspheres were found to embolise in tumour tissue at concentrations of up to 39 times that of the surrounding normal tissue. As a result, adriamycin microsphere therapy was found to retard significantly (P<0.01) tumour growth rates compared to growth rates associated with similar doses of adriamycin delivered as free drug rather than bound to controlled release microspheres. Equivalent sham microsphere treatments showed no significant difference in tumour growth rates compared with the control group. Adriamycin loaded on to ion exchange microspheres holds strong potential for treatment of human malignancy.