Reaction centers from the photosynthetic bacterium Rhodobacter sphaeroides wild type strain 2.4.1 were treated with potassium borohydride. The procedure was found to remove the monomeric bacteriochlorophyll on the M subunit side and also to dislodge the bound carotenoid molecule, spheroidene, from its binding site. Absorption, electron spin resonance and transient optical spectroscopic data on the modified reaction centers are presented. The data are in agreement with a close proximity between the carotenoid and the M-subunit side bacteriochlorophyll indicated by the X-ray analysis of Yeates et al. (1988).