The effect of four kinds of the selenium compounds on Staphylococcus aureus growth was studied by microcalorimetry. The extent and duration of the inhibitory effect on the metabolism as judged from the rate constant (k), varied with the different drugs. The rate constant of S. aureus (in log phase) in the presence of the drugs decreased with increasing concentrations of the drugs. The experimental results reveal that the sequence of antibiotic activity of selenomorpholines is: N,N'-methylene bisselenomorpholine > selenomorpholine and Na2SeO3 > selenomorpholine hydrochloride.