Serum mecillinam concentrations have been obtained in 6 volunteers after intravenous injection of 200 mg mecillinam and two 200 mg tablets containing pivmecillinam hydrochloride. Initial concentrations were between 6 and 9 mg/1 and peak concentrations of about 2-0 mg/1 occurred at 1 to 1-5 h after the tablets. Analysis of the intravenous data shows the concentration/time curve to be biphasic and similar to that previously reported for penicillin G. The biexponential curves describing the data have been calculated using nonlin. Comparison of the areas under the serum mecillinam concentration/time curves suggests a bioavailability of 65 to 70% for the tablets. However, the two preparations contain therapeutically equivalent amounts of antibiotic, as two 200 mg tablets of pivmecillinam contain 30% more mecillinam than the 200 mg injection.