In this paper, we have used inhibitor titrations to reinvestigate the interaction of complexes in chromatophores from Rhodobacter sphaeroides. Measurements which assay the titration of catalytic sites on the complex show stoichiometries for some inhibitors (stigmatellin and antimycin) close to 0.5 mol inhibitor/ mol complex. This behavior is most readily explained in terms of a functionally dimeric bc-complex. We have also investigated the kinetic behavior of the chain on titration with inhibitors. Starting from either a partly reduced or fully oxidized quinol pool, when an inhibitor (stigmatellin, antimycin, myxothiazol or UHDBT) is bound so as to inhibit more than 80% of centers, the remaining fraction of bc-complexes can catalyze the reduction by quinol from the pool of all cytochrome ct (c1 plus c2) oxidized by flash activation. Over the entire range of the titration, the initial rate of reduction of cytochrome ct was linearly proportional to the concentration of uninhibited centers. The results suggest that the oxidation of quinol is second-order, with the rate of reaction determined by a second-order rate constant, the concentration of uninhibited centers, and a concentration of kinetically competent reaction partner (either cytochrome c2 or quinol) reflecting a delocalized pool. It seems probable that the reaction domain is a single chromatophore.