High-resolution site-selection fluorescence- and hole-burning spectroscopy were used to study energy transfer in two LH2 light-harvesting complexes of purple bacteria : the B800-850 complex of isolated Rb. sphaeroides and the B800-820 complex of Rps. acidophila, at 1.2 K. Fluorescence spectra, hole widths, and hole depths were measured as a function of excitation wavelength lambda(exc) within the B800 band. For lambda(exc) greater than or equal to 798 nm, fluorescence line-narrowing is observed and the energy-transfer times (tau = 2.5 and 2.0 ps for B800-850 and B800-820, respectively) are independent of lambda(exc). In this spectral region only interband B800 --> B850 (B820) energy transfer takes place. For 780 nm less than or equal to lambda(exc) less than or equal to 798 nm, the fluorescence bands are bread and the transfer time, obtained from hole widths extrapolated to zero burning-flurence density, decreases toward the blue side of B800. In this wavelength region competition occurs between B800 --> B850 (B820) and B800 --> B800 "downhill" energy transfer. For lambda(exc) less than or equal to 780 nm, the broad fluorescence bands, with maxima at lambda(em) similar to 805 nm, become independent of lambda(exc) and intraband B800 --> B800 transfer combined with excited-state vibrational relaxation are the dominant processes. The spectral distribution of the most-red absorbing pigments within the B800 band, which transfer energy exclusively from B800 to B850 (B820), was determined from the depth of the hole versus lambda(exc). The results indicate that one-third of the B800 pigments transfer their energy only to B850 (B820), from which it is concluded that the minimal functional LH2-unit consists of at least three B800 pigments and six B850 pigments, in addition to carotenoids.