The susceptibility of 83 non-pigmented Serratia marcescens strains was determined by an agar dilution technique. They originated from miscellaneous pathological specimens submitted to the diagnostic laboratory during a nosocomial infection outbreak in 1974. All strains were completely resistant to 128 μg/ml of cephalothin, colistin sulphomethate, lincomycin and penicillin G. They were also resistant to clinically attainable concentrations of ampicillin, chloramphenicol, erythromycin, novobiocin and tetracycline. With regard to drugs with some activity, 84% of the strains were susceptible to nalidixic acid, 48% to sulphamethoxazole, 57% to streptomycin, 60% to kanamycin, 61% to gentamicin, 85% to co-trimoxazole and 100% to amikacin. Environmental strains isolated from the infected units were strikingly more sensitive than the patient strains