The pharmacological and clinical properties of flucloxacillin, a new isoxazolilpenicillin, are reported. This drug shows a low acute and chronic toxicity, and enteric absorption clearly higher than cloxacillin and oxacillin. Its binding capability with serum proteins, though rather high, is nevertheless lower than dicloxacillin. The antibacteric activity of flucloxacillin is particularly evident in Gram-positive bacteria and above all on penicillinases producing staphylococci. Flucloxacillin has proved to have successfull effect in the clinical therapy of several infections, particularly of respiratory apparatus and soft tissues.