Using microbial calorimetry technique, the antimicrobial actions of drugs were quantitatively studied for p-hydroxybenzoic acid alkyl esters (parabens). Parabens testd were p-hydroxybenzoic acid methyl, ethyl, propyl, butyl, pentyl and hexyl esters.<br>For the purpose of obtaining the inhibitory parameters characterizing drug action, growth behaviors of Saccharomyces cerevisiae and Klebsiella pneumoniae grown in media containing various amounts of parabens were observed as each growth thermogram with the multiplex calorimeter having 24 calorimetric units. Based on the growth thermograms of Klebsiella pneumoniae and Saccharomyces cerevisiae, 50% growth inhibition concentrations (K_θs) of each paraben were obtained from the growth retardation times, and then the drug-potency curves were made from the drug efficacy. The minimum growth inhibition concentrations (MICs) obtained by analyzing the drug-potency curves showed that the effects of the drugs become more in the order of methyl, ethyl, propyl, butyl, pentyl, hexyl esters. When comparing the affinity of the drugs to both microorganisms based on the drug concentration required for inhibiting the growth activity by 50%, a linear relationship was observed to exist between the number of carbon atoms in an alkyl chain and the affinity. In addition, an identical drug exhibited a higher effect on S. cerevisiae rather than on K. pneumoniae, which may be mainly due to the difference in membrane structure.<br>From the result obtained it was concluded that the method of analysis developed here is useful for the quantitative study of antimicrobial actions of drugs.