In a steady-state cross-over study in 18 healthy volunteers, the relative bioavailability of beta-escin (CAS 11072-93-8) after oral administration of a new immediate release enteric-coated test formulation containing aesculus extract was evaluated in comparison with a prolonged-release reference preparation. The subject received the test and the reference preparation in randomised sequence for 7 days each with no washout period in between. The daily dose was 50 mg escin b.i.d. Blood samples for pharmacokinetic profiling were taken on the 7th treatment day of each period over a full 24-h cycle of two successive dosing intervals. For the determination of beta-escin serum concentrations, a highly specific radioimmunoassay (RIA) was used. Generally, escin serum concentrations were lower during the second dosing interval (night) than during the first interval, probably indicating a drug by food interaction. (The morning dose was given after overnight fasting whereas the evening dose was given between meals). Test and reference demonstrated bioequivalence with regard to the extent of absorption; for the AUC (0-24 h p.a.), the 90% confidence interval ranged from 84% to 114% (point estimate: 98%). The differences observed for rate parameters can be disregarded due to the generally slow elimination and the wide therapeutic concentration range of escin.