The influence of a fat-rich meal on the pharmacokinetics of cyclosporine from a new oral formulation (Sandimmune Neoral) was compared in a randomized, four-way crossover study to the currently marketed formulation (Sandimmune) in 24 healthy male volunteers. Single oral doses of 300 mg Sandimmune and 180 mg Sandimmune Neoral were each administered once under fasting conditions and once 30 min after starting a high-fat meal. Serial blood samples were obtained over a 48-hr period after each administration, and whole-blood cyclosporine concentrations were determined by a specific monoclonal radioimmunoassay method. Food had a marked effect on cyclosporine absorption from Sandimmune manifested by a nearly doubled time to reach the peak concentration and a 37% increase in the area under the curve. This was associated with significant elevations in subsequent whole-blood cyclosporine concentrations compared to the fasting administration. For Sandimmune Neoral the influence was less pronounced. The maximum concentration was decreased by 26%, without a relevant change in the time to reach the peak; the area under the curve showed a slight reduction of 15%. The relatively minor influence of a fat-rich meal on the absorption of cyclosporine from Sandimmune Neoral is advantageous when individualizing a dosage regimen under clinical and out-patient administration conditions.