A mixture of amoxycillin, semisynthetic penicillin beta-lactamase susceptible, and flucloxacillin, an isoxazolylpenicillin, in combination 1:1 was tested. This mixture shows real synergic effect "in vitro" on beta-lactamase producing Gram-positive and Gramnegative bacteria and an additive effect on the other microrganisms. Good results have been obtained "in vivo" and a protective effect of flucloxacillin on enzymatic degradation of amoxycillin by beta-lactamase was found.