Mutation frequencies to constitutive resistance were determined for 7 strains inducible resistant to the MLS antibiotics (5 Staphylococcus aureus, 1 group G Streptococcus and 1 Streptococcus sanguis). In the 5 staphylococcal strains, mutants were generally selected at a frequency of 10(-9) to 10(-10) (ranging from 4.10(-8) to 9.10(-11) on plates containing 50 mg/l of the following non-inducer MLS: spiramycin, josamycin, midecamycin, miocamycin, lincomycin, clindamycin, and pristinamycin factor I (PI). No mutant was selected by 50 mg/l of pristinamycin factor II (PII) or by pristinamycin complex (P). Absence of selection of constitutive mutant by P was due to the low MICs of the antibiotic against the constitutively resistant strains and to the effect of PII: the emergence of the mutants constitutively resistant to PI was prevented by a 6-hour contact of the culture with PII (50 mg/l). MICs and MBCs of the pristinamycin against 9 constitutive mutants were respectively 2 to 4-fold and 2 to 8-fold greater than that against the wild-strains. In the streptococci, no constituvely resistant mutant was selected. Therefore, the risk of selection of resistant mutants by a non inducer MLS in the course of the treatment of infections due to inducible resistant staphylococcus appeared to be low, especially in the case of pristinamycin.