The light-harvesting complex of Rhodospirillum rubrum was reversibly dissociated into its component parts: bacteriochlorophyll and two 6-kilodalton polypeptides. The dissociation of the complex by n-octyl beta-D-glucopyranoside was accompanied by a shift of the absorbance maximum from 873 to 820 nm (a stable intermediate form) and finally to 777 nm. In the latter state, bacteriochlorophyll was shown to be free from the protein. Complexes absorbing at 820 and 873 nm could be re-formed from the fully dissociated state with over 80% yield by dilution of the detergent. Absorbance and circular dichroism properties of the re-formed B820 complex were essentially identical with those of B820 formed from chromatophores. Phospholipids and higher concentrations of complex were required to obtain the in vivo circular dichroism spectrum for reassociated B873. Reconstitution of the light-harvesting complexes from separately isolated alpha- and beta-polypeptides and bacteriochlorophyll was also demonstrated. Absorbance and circular dichroism spectra of these complexes were identical with those of complexes formed by the reassociation of the dissociated complex. Bacteriochlorophyll and the beta-polypeptide alone formed a complex that had an absorbance at 820 nm, but an 873-nm complex could not be formed without addition of the alpha-polypeptide. The alpha-polypeptide alone with bacteriochlorophyll did not form any red-shifted complex. In preliminary structure-function studies, some analogues of bacteriochlorophyll were also tested for reconstitution.