Antimicrobial susceptibility patterns of 102 clinical isolates of Serratia marcescens from three medical centers were studied by using disk sensitivity and agar dilution methods. The least resistance was demonstrated against gentamicin, nalidixic acid, chloramphenicol, and sulfisoxazole, all of which inhibited more than 80% of the strains. Cephalothin was completely ineffective, and more than 90% of strains were resistant to ampicillin and tetracycline. As demonstrated by the agar dilution method, the minimal inhibitory concentration of nalidixic acid, gentamicin, tobramycin, and chloramphenicol for most strains fell within therapeutically attainable concentrations. The prevalence of resistance to ampicillin, cephalothin, and tetracycline was nearly the same at all three medical centers, whereas there appeared to be patterns characteristic for each center with regard to the other drugs used. Eleven of the isolates produced pigment and exhibited patterns similar but not identical to those of the nonpigmented strains, all 11 being resistant to between three and six drugs. Half of the strains were resistant to five or more antibiotics, indicating that some Serratia exhibit resistance to an unusually broad range of therapeutic agents.