Seventy isolates of Serratia marcescens were obtained from 30 patients in different units of one hospital between April 1982 and February 1983. No common source was found. Not all isolates were multi-resistant and nearly all that were, fell into two main groups, A and B. These groups were defined by phage typing and cephalosporin sensitivity, all apart from one Group B isolate were multi-resistant, whereas Group A isolates contained multi-resistant and sensitive strains. Plasmid screening, resistance transfer studies and plasmid elimination experiments demonstrated that the multi-resistant phenotype was due to a 120 Mdal transmissible plasmid. Resistance to cephalosporins was chromosomally encoded.