The effect of three kinds of the selenomorpholine derivatives on Staphylococcus aureus growth was studied by microcalorimetry. Differences in their capacities to inhibit the metabolism of this bacterium were observed. The growth rate constant, k, of S. aureus (in log phase) in the presence of the drugs decreased with increasing concentrations of the drugs, c. The relationship of k and c is nearly Linear for selenium compounds. Judged from the rate constant k and the half-inhibitory concentration, Ic(50), the experimental results reveal that the sequence of antibiotic activity of selenomorpholines is N-selenomorpholinemethyl succinimide hydrochloride > N-selenomorpholinemethyl succinimide > N-(alpha -selenomorpholinebenzyl) succinimide