The plasma kinetics of platinum after i.v. bolus administration of cisplatin was determined for 17 patients with advanced cancer. Statistical analysis of individual values revealed a high correlation between the area under the plasma concentration-time curve (AUC) of free platinum (unbound to proteins) and the concentration of platinum bound to plasma proteins 24 h after drug administration (Cp24). A similar correlation was found between the peak plasma values of ultrafiltrable platinum (Cp0) and Cp24. When studied in the same patient, increases in free platinum AUC and Cp0 were also found to result in increased Cp24. It is suggested that a single measurement of plasma platinum concentration 24 h after i.v. infusion of cisplatin could be a simple method either of detecting patients with extreme values of AUC and Cp0 or of studying the evolution of these parameters during multiple courses of treatment, although it cannot be used to give precise values for AUC and Cp0.