The bactericidal activities of different types of quaternary ammonium compounds (QAC) were measured in distilled water at 0, 20 or 25°C for concentrations between 0.001% (w/v) and 25% (w/v). Four different gram-negative bacteria were used as test organisms. In a qualitative suspension test, the existence of a concentration for maximum activity was demonstrated, depending on the temperature, and the type of QAC used and the organism tested. In a quantitative test, Pseudomonas aeruginosa and Alcaligenes xylosoxidans 9 isolated from a benzalkonium chloride concentrate, were used as test organisms with different resistances to QACs. Thebactericidal activities of QACs showed a parabolic curve when plotted against concentration. In the case of dodecyl-, tetradecyl- and hexadecyltrimethylammoniumbromide, the optimal concentration for the bactericidal activity of the homologous salts shifted toward lower concentrations as the vicinity of critical micelleconcentration.