Apoptosis or programmed cell death (PCD) is a type of death occurring in various physiological processes. Several data suggest that: (1) apoptosis may play a critical role in AIDS pathogenesis; (2) an increase of endocellular free radical levels can be associated with activation of previously latent HIV virus. Tumor necrosis factor (TNF), a cytokine capable of inducing oxygen free radicals and apoptosis, appears also to be involved in HIV activation. The present findings, which elucidate a relationship between the percentage of apoptotic cells, reduced glutathione (GSH) depletion and an increase of p24 antigenemia, suggest that pretreatment with N-acetylcysteine (NAC) is capable of decreasing the above-mentioned phenomena in HIV-infected U937 cells.