Virginiae butanolide C (VB-C) is an autoregulator which triggers virginiamycin production in Streptomyces virginiae. A new binding assay with tritium-labeled VB-C analogue (2,3-cis-2-(1'-hydroxy-[6',7'-3H]heptyl)-3-(hydroxymethyl)butanolide+ ++ ) was developed and a specific VB-C binding protein was purified to homogeneity from crude extracts of S. virginiae by ammonium sulfate fractionation, DEAE-Sephacel and Sephadex G-100 column chromatographies, hydrophobic HPLC on phenyl 5PW and native polyacrylamide gel electrophoresis. The VB-C binding protein showed an apparent Mr of 35,800 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and Mr of 26,000 approximately 44,000 on native molecular sieve HPLC, indicating the monomeric nature of the binding protein. The binding protein efficiently bound to a VB affinity column and eluted specifically by VB-C, which confirmed the specific nature of the binding protein. The binding activity decreased by 40% in the presence of genomic DNA from S. virginiae, indicating interaction between the VB-C binding protein and the DNA.