The effect of N-acetyl-L-cysteine on the cytotoxicity of tumor necrosis factor-alpha was investigated in cultured bovine pulmonary artery endothelial cells and L929 mouse tumor cells. In endothelial cells, a 72-h incubation with tumor necrosis factor-alpha (100 ng/ml) reduced the number of viable cells to 27% of control. Simultaneous incubation with N-acetyl-L-cysteine (0.5-5 mmol/l) protected endothelial cells from tumor necrosis factor-alpha-mediated cytotoxicity and increased viability in a concentration-dependent fashion to 69% of control. Under the same conditions, a 72-h incubation with tumor necrosis factor-alpha (100 ng/ml) reduced the number of viable L929 tumor cells to 31% of control. However, this cytotoxic response remained unaltered in the presence of N-acetyl-L-cysteine (0.5-5 mmol/l). Similar results were obtained when using a lower concentration of tumor necrosis factor-alpha (50 ng/ml). These findings demonstrate protection from tumor necrosis factor-alpha-mediated toxicity by N-acetyl-L-cysteine in endothelial cells but not in a tumor cell line. It is concluded that N-acetyl-L-cysteine might serve as a therapeutic agent to limit the vascular toxicity of tumor necrosis factor-alpha without affecting its antineoplastic activity.