The pharmacokinetic evaluation of a cross-over study of high intravenous doses of penicillins has been compared with bacterial sensitivity data to achieve dosage recommendations for intravenous infusions aimed at septicaemias. The serum curves were biphasic, with an initial phase of approximately 1 h. Slightly longer half-life values were noted with large doses of the penicillins than found with smaller doses. The highest dosage of ampicillin tested was 2 g which, in severe infections, probably should be given every 4 h. It would be better to infuse 5 g 3–4 times daily. When given for P. aeruginosa, carbenicillin 10 g q.i.d. would usually be sufficient. Flucloxacillin would be more favourable than cloxacillin because it has a marginally better antibacterial effect and a slower elimination. This makes only 4 doses/24 h of flucloxacillin sufficient, whereas 6 doses would be necessary to achieve the same serum levels with cloxacillin. Penicillin G, when given for septicaemias of unknown origin, often in combination with a cephalosporin, should be administered at least 4 times/24 h.