The in vitro activity against Pseudomonas aeruginosa was investigated for serial dilutions of eight anti-Pseudomonas antibiotics in combination with serial dilutions of N-acetylcysteine (NAC) and 2-mercaptoethane sulphonate (MES). The results indicated that addition of NAC and MES to the antibiotics increased the MIC values for tobramycin, netilmicin, piperacillin, azlocillin, cefsulodin, ceftazidime and imipenem but not for colistin. The decrease of MIC found in high concentrations of NAC and MES was probably due to the bacteriostatic effect of the drugs per se and not to synergism with the antibiotics.