Serum samples of 120 patients in different stages of chronic human immunodeficiency virus type 1 (HIV-1) infection, 11 patients with primary HIV-1 infection (PHI), and 49 HIV-1 seronegative homosexual men were analyzed for tumor necrosis factor-alpha (TNF-α), interferon-alpha (IFN-α), and HIV-1 p24 antigen. Increased levels of IFN-α and TNF-α were found in some, but not all, cases with PHI. During progressing disease IFN-α occurred in serum with increasing frequency and concentration. Raised levels of TNF-α were found in all stages of chronic infection, but were less common in patients with AIDS than were raised levels of IFN-α. The levels of the two substances were not correlated. There was a correlation between IFN-α, but not TNF-α, and the occurrence of HIV-1 p24 antigen in serum. These results suggest that IFN-α and TNF-α are induced by different agents during HIV-1 infection. The findings would be consistent with the hypothesis that IFN-α and TNF-α are counteracting forces that have important down- and upregulatory effects, respectively, on HIV-1 replication in vivo.