A controlled double-blind biometric and an open clinical trial were conducted to determine the therapeutic effectiveness of a new equal-dose w/w association of amoxicillin and flucloxacillin. The following conclusions were drawn. Both antibiotics were present in high serum levels; those of flucloxacillin were higher and more persistent. Analysis of variance on 89 patients pointed to the superiority of the association by comparison with amoxicillin alone. The clinical study made it clear that the broad and complementary spectrum of the association, its synergy, absence of toxicity and good gastric tolerance make it a valuable and effective therapeutic aid, also in presence of germs that produce beta-lactase.