Chequerboard and time–kill methods were used to compare the in vitro efficacies of the combinations gatifloxacin (GAT) with cefoperazone (CFP) and GAT with cefoperazone–sulbactam (CFP-SUL) against 58 clinical isolates of Pseudomonas aeruginosa. The combinations GAT+CFP and GAT+CFP-SUL were shown to be synergistic for 36.2 and 58.6 % of isolates tested, respectively, using the chequerboard method. Time–kill studies with 11 strains showed synergy in 54.5 % for the GAT+CFP combination and 72.7 % for the GAT+CFP-SUL combination. The agreement between these two methods was found to be 72–81 %. There was a significant difference in synergy between the two combinations tested (P=0.011).