The pharmacokinetics of chlorambucil, phenylacetic acid mustard (the beta-oxidation product of chlorambucil), and prednisolone were investigated in a cross-over study after oral administration of chlorambucil (30 mg) + prednisolone (50 mg) versus prednimustine (300 mg), the ester of chlorambucil and prednisolone. Intact prednimustine could not be detected in plasma at any time. After administration of prednimustine, the plasma concentration-time curves of chlorambucil, phenylacetic acid mustard, and prednisolone showed a retarded profile compared to the administration of the single components. The mean bioavailability was 14% for chlorambucil, 21% for phenylacetic acid mustard, and 22% for prednisolone, when given as prednimustine, compared to the administration of free compounds in stoichiometrically equivalent doses. When given in the oral dosages mentioned above, the average dose-intensity was 62% for chlorambucil, 95% for phenylacetic acid mustard, and 72% for prednisolone, indicating sufficient therapeutic concentrations of the detectable agents.