The photosynthetic apparatus of the bacterium Rhodobacter sphueroides contains three types of pigment protein: the photochemical reaction centre, and the LH1 and LH2 light-harvesting (LH) complexes. LH1 surrounds and interconnects the reaction centres, forming an LHl-reaction centre (RC) ‘core’ which receives excitation energy from the peripheral LH2 antenna [l]. This arrangement of complexes presents an experimentally attractive model system for the study of the transfer and trapping of light energy in photosynthesis. There are several reasons for this. First, there is a crystallographically determined structure for the RC [2-41. Second, a wealth of spectroscopic data are available on the RC and I,H complexes. Finally, there is a full range of genetic tools that can be used to produce site-directed mutations in the genes encoding these complexes [5, 61. Light energy is harvested by IJH1 and LH2 complexes, which are both composed of highly aggregated states of a and p polypeptides in close association with bacteriochlorophyll and carotenoid pigments. Secondary-structure predictions indicate that these polypeptides span the membrane once, and form an interconnecting network of several hundred pigments which increase the surface area and range of wavelengths over which light energy can be absorbed. Although this network contains only one type of tetrapyrrole pigment, bacteriochlorophyll a, the antenna system is populated with a series of these molecules which absorb at progressively red-shifted wavelengths so that an excited state at the periphery of a photosynthetic ‘unit’ migrates in an energetically ‘downhill’ fashion towards the RC where the energy is trapped and photochemistry takes place. Spectroscopic studies on the antenna of Rb. sphaeroides have been extremely useful in defining the rates of energy transfer between these pigments, and the orientations of these pigments with respect to the plane of the membrane. The availability of mutant strains has also simplified the interpretations of complicated and overlapping spectroscopic signals by contributing a series of strains in which LH2, LH1 or indeed the RC, can be examined in