Plasma ultrafiltrates are routinely used in pharmacokinetic studies of carboplatin. Experiments were performed to detect and quantitate artifactual decreases in the platinum concentration of ultrafiltrates prepared from plasma samples stored at -20 degrees C and -70 degrees C. Carboplatin was added to anticoagulated, whole human blood to produce a 20 microgram/ml concentration. Plasma produced from the blood was stored frozen at either -20 degrees C or -70 degrees C. Aliquots from each storage condition were thawed and ultrafiltered once a week for up to 100 days. Platinum concentrations in ultrafiltrates and plasma were determined by flameless atomic absorption spectrometry. There was no loss of ultrafilterable platinum in plasma samples stored at -70 degrees C, whereas there was a steady decrease in free platinum concentration in ultrafiltrates prepared from plasma samples stored at -20 degrees C. These results imply that pharmacokinetic studies of carboplatin should use ultrafiltrates prepared immediately or that plasma for such studies should be stored at -70 degrees C. Storage of carboplatin-containing plasma at -20 degrees C and subsequent ultrafiltration is not acceptable, because measurement of platinum in such ultrafiltrates will be artifactually low.