A basic, random copolymer of l-ornithine and l-leucine (OL; molar ratios 1:1) was bactericidal to a sensitive (S) strain of Staphylococcus aureus at low concentration. Resistant cells (R) were selected from the culture medium and, after serial transfers to solutions containing increasing amounts of the polymer, grew well in the presence of very high concentrations of it (1,000 mug/ml). S cells bound much more OL than did R cells, but no difference in binding was shown between separated cell walls or cell membranes of S and R. The binding of OL and sensitivity to it were not dependent on the teichoic acid-content of the cells. Bound OL was only partially removed from the cells by a variety of reagents, such as sodium dodecyl sulfate, Triton X-100, dilute trichloroacetic acid, and Ba(OH)(2), and the extent of removal was similar for R and S cells.