Exposure of Serratia marcescens cells to 10, 5, and 2.5 μg/ml of polymyxin B resulted in outer cell surface alterations that consisted of coarse, pleomorphic projections which revealed a double-contoured membrane structure. In contrast, fresh, but not heat-inactivated human serum caused the deposition of very fine, thread-like aggregates on the outer cell surface of exposed cells. The combination of polymyxin B and fresh human serum caused clearly discernible ultrastructural changes of the polymyxin and the fresh serum type, respectively