The core complex formed by the reaction center and the light harvesting complex 1 (RC-LH1) was purified from the photosynthetic bacterium Rhodobacter sphaeroides. We analyzed the lipid and ubiquinone (UQ) complements copurifying with the RC-LH1 complex and with the peripheral antenna (LH2). In RC-LH1 UQ was almost ten times more concentrated than in the LH2 and in the native membranes from which the complexes were extracted. The fractional lipid composition of the RC-LH1 complex also differed from that of the intact membranes, exhibiting a marked increase in cardiolipin concentration. We propose that the confinement of cardiolipin and ubiquinone observed within the RC-LH1 complex, plays a role in vivo in the stabilization of the light-induced charge separation catalyzed by the RC.