This work is devoted to investigation of regularities of mass-energy balance of purple bacteria photorophic growth. Estimates of culture growth efficiency were derived from bioenergetic properties of cell metabolism. An equation system describing joint balance of flows of metabolite reductivity, high-energy protons (HEP) and high-energy bonds (HEB) was formulated. The maximum cell yield from light and the rate of energy expenditure for cell maintenance were found as mathematical expressions containing metabolic stoichiometric coefficients, rates of energy losses and intracellular matter turnover. Experimental study was conducted with a purple bacterium Rhodobacter capsulatus grown continuously on hydrogen at different illumination intensities. The obtained numerical estimates of attainable maximum growth yield were compared with experimental data both taken from literature and obtained in this work. Theoretical and experimental yield values showed good agreement. (C) 2003 Elsevier Ltd. All rights reserved.