A new system for the local delivery of chemotherapy to malignant solid tumors has been developed based on calcium phosphate (CaP) nanoparticles. The adsorption of the anti-neoplastic drug cis-diamminedichloroplatinum (cisplatin) was characterized on three types of apatitic CaP (poorly and well crystallized hydroxyapatite, and carbonated apatite). Adsorption isotherms obtained in chloride-free phosphate solutions at pH = 7.4 (24 and 37 degrees C) indicate that cisplatin adsorption increases with temperature and increases with decreasing crystallinity. Release studies in phosphate buffer saline (containing the chloride ion essential for release) showed that while the cumulative amount of released drug was the same for all apatites at 20 days (approximately 70% of the total bound), the least crystalline material released the drug more slowly. The drug release rate increased slightly with temperature. Cytotoxicity testing was conducted in a K8 clonal murine osteosarcoma cell line to verify that drug activity was retained after adsorption onto the apatite crystals. K8 cells were plated onto dried films of the apatite/cisplatin conjugates and after 24 h, viability was measured with tritiated uridine. The apatite/cisplatin formulations exhibited cytotoxic effects with a dose dependent diminishment of cell viability.