Physiological effects of phenoxyacetic acid, the penicillin V side-chain precursor, on steady-state continuous cultures of Penicillium chrysogenum have been studied both theoretically and experimentally. Theoretical calculations show that at an extracellular pH of 6.50, phenoxyacetic acid has negligible influence on the growth energetics due to protonophoric uncoupling of membrane potentials by passive diffusive uptake. On the other hand, when the extracellular pH is lowered to 5.00, a severe maintenance-related uncoupling effect of phenoxyacetic acid is calculated. These findings were confirmed experimentally by steady-state continuous cultivations with a high-yielding penicillin strain of P. chrysogenum performed on a chemically defined and glucose-limited medium at pH 6.50 and pH 5.00, both with and without phenoxyacetic acid present. The yield and maintenance coefficients were determined from steady-state measurements of the specific uptake rates of glucose and oxygen and the specific production rate of carbon dioxide as functions of the specific growth rate. Combining these data with a simple stoichiometric model for the primary metabolism of P. chrysogenum allows quantitative information to be extracted on the growth energetics in terms of ATP spent in maintenanceand growth-related processes, i.e. m ATP and Y xATP . The increased maintenance-related ATP consumption when adding phenoxyacetic acid at pH 5.00 agrees with the theoretical calculations on the uncoupling effect of phenoxyacetic acid. When Y xATP is compared with earlier reported values for the theoretical ATP requirement for biosynthesis of P. chrysogenum, i.e. Y xATP,growth , it is found that Y xATP,growth is only 40-50% of Y xATP , which stresses that a large amount of ATP is wasted in turnover of macromolecules, leaks, and futile cycles.