OBJECTIVE To compare the bioavailability of cyclosporine from two oral dosage forms of a new microemulsion formulation. METHODS Open, randomized, three-treatment, three-period crossover investigation. METHODS University-affiliated clinical pharmacology research unit. METHODS Twenty-four healthy male volunteers. METHODS Single oral administrations of cyclosporine 180 mg given as a soft gelatin capsule (reference), an oral solution under fasting conditions, and the oral solution mixed with orange juice. RESULTS Serial venous blood samples were obtained over 48 hours after each administration to measure cyclosporine in whole blood by a specific monoclonal radioimmunoassay. For all three treatments, the mean maximum blood concentration (Cmax) of approximately 1100 ng/ml was reached at about 1.3 hours (tmax) after administration; the area under the blood concentration-time curve (AUC) was, on average, 4700 ng.hr/ml. Bioequivalence was conclusively demonstrated for both the absorption rate (Cmax and tmax) and extent (AUC) of cyclosporine among the treatments inasmuch as the point estimates and 90% confidence intervals were within the respective equivalence ranges. CONCLUSIONS When administered in conjunction with routine concentration monitoring, the two oral dosage forms of the new microemulsion formulation of cyclosporine can be interchanged without need for dosage adjustments. In addition, the oral solution can be mixed with fruit juice without affecting the rate or extent of cyclosporine absorption.