Combined resistance against chloramphenicol (CHLOR), nalidixic acid (NA), and trimethoprim (TMP) was demonstrated in isolates and variants of a nosocomially significant, multiple-drug-resistant, nonconjugative strain of Serratia marcescens (bacteriocin type 18), either as a spontaneous event or after exposure of isolates and variants derived therefrom to increasing concentrations of CHLOR, NA, and TMP, respectively. This phenomenon was also noted among several selected mutants of two control strains of S. marcescens. The level of resistance was not absolute; the combined resistant mutants and variants were inhibited by 100–200 μg/ ml of CHLOR, 25–50 μg/ml of NA, and 12.5–25 μg/ml of TMP. However, the phenomenon of combined triple-resistance occurred irregularly with respect to the strain or variant of S. marcescens employed, and with regard to the selective drug utilized. Selection of mutants with CHLOR yielded a larger number of combined CHLOR-NA-TMP resistant mutants than selection with NA or TMP. Preliminary data obtained with spontaneously ‘cured’ variants of the multiple-drug-resistant strain of S. marcescens that had lost resistance against CHLOR, NA, TMP, penicillins, and aminoglycoside antibiotics, and which subsequently were shown to yield spontaneous mutants with combined triple-resistance against CHLOR, NA, and TMP, seem to indicate that this resistance phenomenon is not mediated extra-chromosomally, but rather chromosomally, and subject to phenotypic variation. The precise nature of this novel, unusual resistance mechanism against these three unrelated antimicrobial drugs remains to b