Resistance patterns to antibiotics differed markedly between indole-positive and indole-negative Klebsiella pneumoniae strains. Multiple-drug-resistant strains were almost exclusively indole-negative. Ampicillin and kanamycin resistances in the indole-positive strains tested were not transmissible, whereas many of those resistances in the indole-negative strains were transmissible, together with other drug resistances to an Escherichia coli recipient. The substrate profile of the β-lactamase from the indole-positive strains was fairly different from that of the β-lactamase mediated by the ampicillin resistance plasmi