Antagonism by an antibiotic-producing Staphylococcus epidermidis strain (S6+) against other bacteria was studied semi-quantitatively on solid media and quantitatively in liquid batch culture. Parallel experiments were performed with a similar but non-antibiotic producing strain (S6-). On solid media strain S6+ in the logarithmic growth phase unilaterally inhibited members of all gram-positive species tested, whereas S6- had no effect. In liquid media, more complex interactions were observed, including mutual and reversed antagonisms. The relevance of these findings to the ecology of human skin is discussed.