Imipenem and amikacin, alone and in combination, were investigated for their postantibiotic effect (PAE) on Pseudomonas aeruginosa. Four clinical strains of P. aeruginosa in the logarithmic phase of growth were exposed for 1 h to antibiotics, alone and in combination. Recovery periods of test cultures were evaluated after dilution using viable counting. Imipenem produced a PAE ranging from 0.7 to 1.55 h. Similar PAEs were induced by amikacin (ranging from 0.65 to 2 h). In combination, imipenem and amikacin produced as a final PAE (ranging from 1.6 to 2.65 h), a rough mathematical sum of the individual effects. The finding of this study may have important implications far the timing of doses during therapy with antimicrobial combinations.