The effect of quaternary ammonium surfactants C10H21N(CH3)(3)I, C12H25N(CH3)(3)Br, C14H29N(CH3)(3)Br and C16H33N(CH3)(3)Br, and cetylpyridinium chloride (C16H33)PYCl, on the inhibition of the aerobic metabolism of Saccharomyces cerevisiae was studied with a flow calorimeter. All the compounds, except C10H21N(CH3)(3)I, presented a linear dependence of the biological response (BR) with the logarithm of the concentration (log(dose)). The values of (dose)(max) has an excellent linear correlation with the number of CH2 groups. The bioactivity of these compounds follows the order (C16H33)PYCl >> C16H33 N(CH3)(3)Br > C14H29N(CH3)(3)Br > C12H25N(CH3)(3)Br. Decyltrimethylammonium iodide did not affect the aerobic metabolism of S. cerevisiae under the studied conditions. (c) 2008 Elsevier B.V. All rights reserved.