Mecillinam (FL 1060) is a new beta-lactam antibiotic particularly active against gram-negative organisms. When given intravenously, very high serum levels were maintained for a short period of time. Lower peak levels but comparable bioavailability were obtained after intramuscular administration. Gastrointestinal absorption of mecillinam is poor, and for effective oral therapy the drug must be given in the form of its pivaloyl-oxymethyl ester pivmecillinam (FL 1039) which is well absorbed and rapidly transformed to mecillinam by enzymatic hydrolysis in the body. Urinary recovery of mecillinam after orally administered pivmecillinam was 45% in the first 6 h compared with 55 and 59% after mecillinam given by the intravenous and intramuscular routes, respectively. By increasing the dose the orally active ester produced proportionally higher levels of mecillinam, and the area under the serum curve was doubled with the dose. Higher peak levels and prolonged maintenance of high serum concentrations were seen after administration of pivmecillinam with probenecid. The presence of food in the stomach did not influence the absorption of pivmecillinam to any great extent.