The fusidic acid steady-state concentrations in serum (S) and skin suction blister fluid (SBF) after oral doses of 250 and 500 mg administered twice daily for 6 days, as film-coated tablets of sodium fusidate, were studied in eight healthy subjects. The mean peak serum concentrations after the 250- and 500- mg regimens were 39 +/- 5 and 102 +/- 11 mg/l, respectively, obtained between 2 and 3 h after drug intake. The corresponding values for SBF, obtained later, between 2 and 12 h after drug intake, were 21 +/- 5 and 79 +/- 11 mg/l. As measured by the ratio of area under the concentration vs. time curve (SBF/S), fusidic acid penetration is 69-75%, whatever the dose. With either regime, the fusidic acid SBF and serum levels exceed the minimal inhibitory concentration of bacteria usually found in skin infections, especially Staphylococcus species. A dose of 250 mg twice a day appears sufficient to treat these infections and could be tested in clinical studies.