Triton X-100 solubilized monomers of bacteiorhodopsin (bR) show a decrease in the extent of light adaptation; the red shift and the absorbance increase of the visible absorption band are reduced no less than half the values observed in purple membrane (p.m.) with a corresponding reduction in the isomerization of 13-cis- to all-trans-retinal. Cross-linking of bR with glutaraldehyde before exposure to Triton prevents dissociation of the lattice and reduction in light adaptation. Experiments with cross-linked and lipid-extracted p.m. show that Triton effectively substitutes for the native membrane lipids and that the lattice structure apparently stabilizes the light-adapted state of bR under illumination. In lipid vesicles at molar lipid protein ratios greater than or equal to 80, bR exists as monomers above the lipid-phase transition and aggregates below the phase transition. Above the lipid-phase transition and aggregates below the phase transition. Above the lipid-phase transition light adaptation in the monomers, measured as either the red shift of the visible absorbance maximum or the isomerizaiton o 13-cis- to all-trans-retinal, is also reduced to less than half of the extent observed in intact purple membrane or in the bR aggregates formed in lipid vesicles below the plhase transition. At very high lipid to protein ratios, bR molecules cannot aggregate when the temperature is decreased below the phase transition, and these monomers in a solid lipid phase show the same reduced extent of light adaptation as monomers above the phase transition, thus confirming that this effect is mainly due to the absence of protein-protein interaction and not to the state of the lipid. The extent of the red shift upon light adaptation may be used as a convenient indicator to distinguish the aggregated and monomeric states of bR.