Photoheterotrophic and heterotrophic suspension cultures of tobacco (Nicotiana tabacum L.) were grown with I mM glutathione (reduced; GSH) as sole source of sulfur. Addition of sulfate to both cultures did not alter the rate of exponential growth, but affected the removal of GSH and sulfate in different ways. In photoheterotrophic suspensions, addition of sulfate caused a decline in the net uptake of GSH, whereas sulfate was taken up by the green cells immediately. In heterotrophic suspensions, however, addition of sulfate did not affect the net uptake of GSH and sulfate was only taken up by the cells after the GSH supply in the medium had been exhausted. Apparently, GSH uptake in photoheterotrophic cells is inhibited by sulfate, whereas sulfate uptake is inhibited by GSH in heterotrophic cells. The differences in the effect of GSH on sulfate uptake in photoheterotrophic and heterotrophic tobacco suspensions cannot be attributed to differences in the kinetic properties of sulfate carriers. In short-time transport experiments, both cultures took up sulfate almost entirely by an active-transport system as shown by experiments with metabolic inhibitors; sulfate transport of both cultures obeyed monophasic Michaelis-Menten kinetics with similar app. K m (photoheterotrophic cells: 16.0+2.0 gM; heterotrophic cells: 11.8 + 1.8 gM) and Vma x (photoheterotrophic cells: 323 + 50 nmol. min-1. g-1 dry weight; heterotrophic cells" 233 + 3 nmol.min-1.