Homozygous human immunodeficiency virus type 1 (HIV-1)-transgenic mice (Tg26) appear normal at birth but die within 3 to 4 weeks. The skin of these animals shows diffuse scaling and high-level expression of both HIV-1 mRNA and gp120. Previous experiments showed that treatment with human chorionic gonadatropin (hCG) prevented death and the expression of HIV-1 mRNA and gp120. The present experiments were initiated to study the role of tumor necrosis factor alpha (TNF-α) in HIV-1-induced pathology. Examination of the sera of Tg26 mice revealed a 50-fold increase in TNF-α levels compared to those in nontransgenic mice. Treatment with antibody to TNF-α prevented death, resulted in near normal growth, and produced a marked decrease in skin lesions and a profound reduction in the expression of HIV-1 mRNA and gp120. Both TNF-α antibody and hCG reduced TNF-α levels in sera by approximately 75%. We conclude that TNF-α contributes in a major way to HIV-1-induced pathology in transgenic mice and that both hCG and antibody to TNF-α prevent the development of pathology by suppressing the level of TNF-α.