The pharmacokinetics of vinblastine in humans was examined using a radioimmunoassay specific for both the Vinca alkaloids and aromatic ring [3H]vinblastine. The data were consistent with a three-compartment open model system with the following values, alpha phase: t1/2=3.90+/-1.46 min; Vc=16.8+/-7.1 liters. beta phase:t1/2=53.0+/-13.0 min; Vbeta=79.0+/-52.0 liters; gamma phase:t1/2=1173.0+/-65.0 min; Vgamma=1656.0+/-717.0 liters. Most significant was the finding that vinblastine is metabolized to deacetylvinblastine and that this compound is more biologically active on a weight basis than the parent. No other biologically active metabolites appeared to be present in urine or in stool.