The absorption and distribution of mecillinam and pivmecillinani was studied in man and animals. Dogs showed high concentrations in serum, kidneys, liver, lungs, urine and bile following 50 mg mecillinam per kg body wt. Pregnant rats and rabbits had low but detectable concentrations in the foetuses. Administration to lactating cows gave very low levels of the drug in the milk. 200 mg Mecillinam intravenously into human subjects produced a mean peak concentration of 12 μg/ml. Intramuscular administration produced later and lower peak serum levels (6 μg/ml). Following 400 mg the peak concentrations were respectively 27 μg and 13 μg/ml. Bioavailability was the same irrespective of which parenteral route was employed. Administration of oral pivmecillinam shows that Cmax values increased by 50% on doubling the dose, and AUC values doubled directly proportionally with the dose. Tmax was delayed following 800 mg which suggested that a threshold value for the absorption mechanism had been exceeded. Absorption of pivmecillinam was found to be unaffected by the presence of food in the stomach.