The pharmacokinetics of vinblastine were studied in 16 patients with non-small cell lung cancer after a bolus intravenous dose of 3 mg/m2 given before or after cisplatin (100 mg/m2). Venous blood was collected at 0, 10, and 36 hr for analysis by radioimmunoassay. The mean plasma vinblastine concentration at 10 hr was similar when vinblastine was given before (4.8 ng/ml; 95% CI, 3.2-6.3) or after cisplatin (4.9 ng/ml; 95% CI, 2.7-7.1). Plasma vinblastine concentrations in patients given cisplatin were higher than previously reported in patients given vinblastine alone. Patients with plasma vinblastine concentrations less than 2. 75 ng/ml at 10 hr experienced less severe neutropenia (37% fall in neutrophil count; 95% CI, 18-55) than those with levels greater than 2.75 ng/ml (69% fall in neutrophil count; 95% CI, 62-77). In conclusion, the pharmacokinetics of vinblastine predict the severity of neutropenia and may be altered when given in conjunction with cisplatin.