A-factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone), produced in a growth-dependent manner, switches on secondary metabolite formation and morphological differentiation in Streptomyces griseus, presumably by binding to the A-factor receptor protein (ArpA)-DNA complex and releasing the repression caused by ArpA. In the A-factor-deficient mutant strain S. griseus HH1 a large deletion includes afsA which is required for A-factor production. Growth and aerial mycelium formation of strain HH1 on media containing high concentrations of sucrose, sorbitol, mannitol, KCl or NaCl was disturbed by the presence of a large amount of A-factor supplied either exogenously or by a high-copy-number plasmid carrying afsA. This disturbance did not occur on media of normal osmolality and was observed only when A-factor was supplied during the very early stage of growth, about 8 h after inoculation. In addition, neither the wild-type strain nor S. griseus KM7 defective in ArpA exhibited the disturbance. These observations suggest that the presence of a large amount of A-factor during the very early stage of growth, probably during the A-factor-sensitive stage, triggered abrupt and disordered expression of some genes. The effect was apparently mediated through ArpA in the A-factor regulatory cascade and disturbed the physiology of strain HH1 under high osmolality. A gene that suppressed the disturbance was identified 5.5 kb upstream of the afsA locus in the wild-type strain. The gene, named sgaA, encoded a protein of 264 aa with a calculated molecular mass of 28 kDa.