The organization of proteins in the chromatophore membrane, particularly of the reaction center and the light-harvesting polypeptide, was examined by the use of a hydrophobic and a hydrophilic cross-linking reagent, namely DSP (dithiobis-succinimidyl propionate) and glutaraldehyde. The linkage of proteins was studied by SDS polyacrylamide pore gradient electrophoresis. DSP was shown to link proteins within the core of the membrane. The subunit H of the reaction center is linked with DSP at a low concentration, either with itself or with other membrane proteins but not to the subunits M and L. In isolated reaction centers the subunits H are exclusively linked with each other. With increasing concentrations of DSP the bands of the subunits M, L, and the light-harvesting polypeptide disappear simultaneously from the gel, suggesting that these proteins are linked together. This hypothesis is supported by the finding that reaction centers isolated from chromatophores treated with DSP retain an appreciable amount of light-harvesting polypeptide. With increasing concentrations of the hydrophilic cross-linking reagent glutaraldehyde, the bands of all the three subunits of the reaction center, H, M, and L, progressively disappear from the gel, suggesting that they are linked together. The light-harvesting polypeptide remains free when this reagent is used.