Photosynthetically active reaction center complexes were prepared from the green sulfur bacterium Chlorobium vibrioforme NCIMB 8327, and the content of quinones was determined by extraction and high-performance liquid chromatography. The analysis showed a stoichiometry of 1.7 molecules of menaquinone-7/reaction center. No other quinones were detected in the isolated reaction centers, whereas membrane preparations also contained chlorobiumquinone. The possible involvement of quinones in electron transport was investigated by electron paramagnetic resonance (EPR) spectroscopy. A highly anisotropic radical was detected by Q-band EPR spectroscopy in both membranes and isolated reaction centers following dark reduction with sodium dithionite and photoaccumulation at 205 K. At 34 GHz, the EPR spectrum is characterized by a g tensor with gxx = 2.0063, gyy = 2.0052, gzz = 2.0020 and delta B of 0.7 mT, consistent with its identification as a quinone. This spectrum is highly similar in terms of g values and line widths to photoaccumulated A1- in photosystem I of Synechococcus sp. PCC 7002. The results indicate that menaquinone-7 in the green sulfur bacterial reaction center is analogous to phylloquinone in photosystem I.