The amino acid residues of the light-harvesting protein B870-alpha of Rhodospirillum rubrum G-9+ that in the chromatophore membranes are in contact with the lipid phase were identified by hydrophobic photolabeling. Three reagents have been used which all contained the trifluoromethyldiazirinylphenyl group as a photo-sensitive precursor of a carbene but which otherwise differed in shape, molecular structure, and in the way they interact with membranes. 3-Trifluoromethyl-3-(m-[125I]iodophenyl)diazirine is a highly lipid-soluble compound, 11-[4-[(trifluoromethyl)diazirinyl]-phenyl]-[10-3H] 9-oxaundecanoic acid is an analogue of a fatty acid, and 1-palmitoyl-2-[11-[(trifluoromethyl)diaziri-nyl] phenyl]-[10-3H]9-oxaundecanoyl]-sn-glycero-3-phosphorylcholine one of a lecithin. Following labeling of chromatophores with these reagents, B870-alpha was isolated and subjected to (solid phase) Edman degradations in order to determine individual amino acid residues labeled. The main features of these results are as follows. 1) Labeling occurred both within the N-terminal segment (residues 1-8) and within the predominantly hydrophobic transmembrane stretch (residues 14-33). 2) Label distribution patterns within segments are indicative of helical structures to which the reagents had access to one face only of the cylindrical envelopes. 3) With regard to the transmembrane segment, the label distribution patterns were similar for all reagents whereas striking differences were noticed within the N-terminal portion. The labeling patterns are consistent with previous models proposing tight association of the transmembrane helix with that of the B870-beta chain. They also suggest that the N-terminal segment forms an amphipathic helix which interacts with the water-lipid interface of the membrane.