Mecillinam is a new amidinopenicillin which is taken orally as its ester pivmecillinam. 10 healthy volunteers were each given 200 mg of pivmecillinam hydrochloride four times daily and all urine was collected and cooled to -20°C to minimize antibiotic degradation. 43% of the ingested antibiotic was recovered from the urine in its micro-biologically active form, during the first 12 h and 34% during the subsequent 24 h. The corresponding figures for 10 volunteers receiving double this dose of antibiotic was 30 and 34%. The mean half life of mecillinam in urine due to spontaneous breakdown at 37 °C was 17 h. There was evidence that stearate-film-coated pivmecillinam hydrochloride tablets may be less likely to cause dyspepsia than gelatine capsules.