The reaction between cisplatin and human serum albumin (HSA) was investigated by X-ray crystallography and crystal structures of the cisplatin/HSA adduct were eventually solved for the first time. Structural data unambiguously prove that cisplatin mainly binds to His105 and Met329 side chains; additional binding sites are detected at His288, Met298, and Met548 and at His535, His67 and His247.