The effect of the antibiotic, pseudomonic acid, on the major metabolic processes in Staphylococcus aureus was studied. The primary effect of low concentrations of the antibiotic, leading to bacteriostasis, is inhibition of protein synthesis. Pseudomonic acid also severely inhibits RNA synthesis which can be prevented by chloramphenicol treatment. DNA and cell wall peptidoglycan synthesis are inhibited to a lesser extent and interference with these processes is considered to be a secondary effect. Oxidative phosphorylation as measured by ATP levels was not inhibited.