Forty beta-lactamase producing Gram-negative organisms were tested with mezlocillin plus an isoxazolyl penicillin for synergy. With standard chequerboard techniques, the combination showed enhanced activity, when compared to either drug alone, against 30 strains. Eighteen of these 30 strains showed synergism at concentrations of mezlocillin and isoxazolyl penicillins which were attainable in the serum (7 strains of Morganella morganii, 8 strains of Eschericia coli and 3 strains of Serratia marcescens).