14 of 74 test strains of Serratia marcescens yielded reproducible cocarde-like growths (coc+) around 30-μg disks of polymyxin B (PB) on Müller-Hinton, brain heart infusion and tryptic soy agar. The coc+ phenomenon was not due to nutrient effects of growth medium nor did it correlate with either group A (phage tail) bacteriocinogeny or colicinogeny as determined with 32 selected test strains; mitomycin C failed to give rise to coc+ growths. The anionic bile salts of MacConkey agar as well as aqueous sodium deoxy-cholate neutralized the coc+ activity of PB. Benzalkonium chloride, chlorhexidine diglu-conate, and cetyltrimethylammonium bromide by themselves did not produce cocardes. Rather, these cationic detergents enhanced PB activity somewhat against selected coc+ and coc–– strains of 5. marcescens. It was concluded that the PB-induced growth phenomenon of S. marcescens was due to the cationic detergent-like activity of this polypeptide antib