In Streptomyces virginiae, production of virginiamycin is triggered by signal molecules named virginiae butanolide A, B or C (VB-A, B or C: Yamada, Y. et al. J. Antibiotics 40: 496-504, 1987). We have found a specific VB-C binding protein from S. virginiae, and characterized it by using a tritium-labeled VB-C analogue as a ligand. By equilibrium dialysis in the absence and presence of radio-inert VB-C, a crude extract from 1 g of wet mycelia specifically bound 3.5 pmol of [3H]VB. The binding disappeared after pronase digestion and showed ligand specificity toward cis VB-C (cis VB-C greater than trans VB-C much greater than A-factor type), indicating that binding was due to a cis VB-C specific binding protein. Scatchard analysis of the binding demonstrated a single class of high affinity binding sites (Kd 1.1 nM) and low number of the binding sites (30-40 sites/genome DNA). By gel filtration on Sephadex G-75 and molecular sieve HPLC, the binding protein was shown to have an Mr of about 20,000. These results indicate that the substance is a novel VB-C binding protein and suggest that it is a VB-receptor mediating the pleiotropic signal transmitted by VBs in S. virginiae.