The effect of cefsulodin in combination with mecillinam was examined against a wide range of bacterial species. The antibacterial spectrum was widened by the combination of cefsulodin and mecillinam in the ratio of 5: 1 and 10: 1. In overall observations, in the in vitro test, a synergistic effect against clinical isolates was found on Klebsiella pneumoniae, Citrobacter freundii, Enterobacter cloacae, Serratia marcesscens, Proteus mirabilis and Proteus vulgaris, and an additive effect was found on Staphylococcus aureus, Escherichia coli, Proteus morganii, Proteus rettgeri, Proteus inconstans and Pseudomonas aeruginosa. In in vivo tests, a synergistic effect was observed on S. marcescens TN 66 and K. pneumoniae DT infections and an additive effect was observed on S. aureus 308 A-1, E. coli O-111 and T-7, C. freundii TN 518, E. cloacae TN 603, P. vulgaris GN 4712, P. morganii TN 373 and P. aeruginosa U 31 infections.