The effects of tumor necrosis factor-alpha (TNF) on glutamine GLN transport by cultured human fibroblasts were studied. Uptake of 3H-GLN was assayed in both the presence and absence of sodium in order to differentiate Na(+)-dependent and Na(+)-independent transport systems. GLN transport was linear (r = 0.99) for at least 15 min and occurred predominantly via a single Na(+)-dependent pathway, consistent with System ASC. Incubation of fibroblasts with TNF (1000 units/ml) for 12 hr resulted in a significant decrease in system ASC-mediated glutamine transport activity. TNF did not alter cell morphology or protein content. Kinetic studies indicated that the decrease in carrier-mediated Na(+)-dependent GLN transport was not due to a change in transporter affinity (Km = 117 +/- 23 microM in controls vs 86 +/- 23 microM in TNF, P = NS), but instead to a 45% decrease in maximal transport rate (Vmax = 4088 +/- 354 pmole/mg protein/30 sec in controls vs 2230 +/- 510 in TNF, P less than 0.05). TNF also decreased Na(+)-independent transport by 50% (mean uptake of 50 microM GLN = 94 +/- 13 pmole/mg protein/30 sec in controls vs 46 +/- 6 in TNF, P less than 0.02). In human fibroblasts, the activity of System ASC, which has generally been viewed as a hormonally unresponsive carrier, is decreased by TNF. This impairment in glutamine transport may result in inadequate amounts of intracellular glutamine to support fibroblast metabolism and possibly function.