During its stationary phase, Streptomyces ambofaciens produces the macrolide antibiotic spiramycin, and has to protect itself against this antibiotic. Young mycelia, not yet producing spiramycin, are sensitive to it, but they become fully resistant when production begins. In a sensitive mycelium, resistance could be induced by exposure to sub-inhibitory concentrations of spiramycin, and these induced mycelia, like producing mycelia were resistant not only to spiramycin but also to several other macrolide antibiotics. Ribosomes extracted from these resistant mycelia were shown in vitro to be more resistant to spiramycin than ribosomes extracted from sensitive mycelium, indicating that S. ambofaciens possesses a spiramycin-inducible ribosomal resistance to spiramycin and to macrolide antibiotics. Studies with spiramycin non-producing mutants showed that, in these mutants, resistance to spiramycin also varies during cultivation, in that an old culture was much more resistant than a young one. But with these non-producing mutants, the spectrum of resistance was narrower, and in vitro data showed that resistance was not due to ribosomal modification. These results suggest that S. ambofaciens presents at least two distinct mechanisms for spiramycin resistance; a spiramycin-inducible ribosomal resistance, and a second resistance mechanism which might be temporally regulated and which could involve decreased permeability to, or export of, the antibiotic. The two mechanisms are probably at work simultaneously in the producing mycelium, the spiramycin-inducible resistance being induced by endogenous spiramycin. In non-producing mutants, in the absence of self-induction by spiramycin, only the second mechanism is observed.