We describe a method of analysis for free circulating platinum species derived from cis-dichlorodiammineplatinum(II) in blood plasma. Protein-bound and free platinum species were separated from each other by centrifugal ultrafiltration. Platinum in the ultrafiltrate was converted to a cationic complex by reaction with ethylenediamine, and the product was collected on paper impregnated with cation-exchange resin, where it could be stored indefinitely without loss. The platinum was eluted from the disk with 5 mol/liter hydrochloric acid, and an aliquot of this solution was then analyzed by flameless atomic absorption spectrophotometry. The overall analytical recovery of platinum was 80 +/- 2%. The minimum quantity of cis-dichlorodiammineplatinum detectable was 35 microgram/liter of plasma at the 99% confidence level. Detector response was linearly related to drug concentration in the range from 80 microgram to 290 mg of Pt per liter of plasma. Reaction variables were made optimal, so as to yield maximum sensitivity and reproducibility (+/- 2%) consistent with minimal sample transfers and manipulations.