SUMMARY: Single-step erythromycin-resistant strains of 56188 fell into two classes: whereas class A mutants were sensitive to lincomycin and carried mutations transducible by phage A 25, class B mutations conferred lincomycin cross-resistance and failed to be transducible under the conditions employed. Recombination and cotransduction experiments allowed the assigning of at least 10 of 13 class A mutations to the same linkage group. From interclass crosses recombinants emerged at frequencies comparable with those at which the donor markers were integrated into wild type. The resistance to three macrolides and lincomycin of interclass recombinants, carrying mutations which were not cotransducible, was related to the resistance levels determined by the individual mutations. The pheno-typic properties of the mutants together with their genetic relationship lead to the conclusion that there are at least two loci governing resistance to erythromycin in strain 56188 of


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