Microcalorimetry was used to study the kinetics of action of five kinds of Schiff base on a strain of Aerobacter aerogenes. Differences in their capacities to suppress the metabolism of this bacterium were observed. The extent and duration of the inhibitory effect on the metabolism as judged from the multiplication rate constant, k, varied with the different Schiff bases. The multiplication rate constants, k, of Aerobacter aerogenes (in log phase) in the presence of Co(III)-NG and Ni-NG (NG : D-glucosamine-beta-naphthol aldehyde) decreased with the increasing concentrations of the compounds C, but the relationship between k and C was not of good linearity. For Fe(III)-NG, the multiplication rate constants are constant irrespective of variations in concentration. Similarly over the concentration range 50-200 mu g mL(-1,) there is nearly change in the inhibitory effects of NG on Aerobacter aerogenes, while they are slightly reduced beyond 200 mu g mL(-1). The experimental results revealed that the sequence of antibiotic activity of Schiff base drugs is : Co(III)-NG > Ni-NG > NG > Fe(III)-NG.