After initial infection with human immunodeficiency virus 1 (HIV-1), patients may remain asymptomatic for years before the onset of acquired immune deficiency syndrome (AIDS). This non-aggressive or latent phase may be manifested by functional abnormalities of both T and B cells, even in the absence of critical reductions in lymphocyte numbers. At present, it is not clear whether the immune abnormalities in either the asymptomatic phase or in clinical AIDS are due solely to direct effects of HIV-1 or whether they also reflect host immunoregulatory mechanisms. In this article, by Charles Via, Herbert Morse and Gene Shearer, the immune abnormalities associated with early HIV-1 infection are compared with immune abnormalities found in three murine models of autoimmunity and immunodeficiency, and it is suggested that host mechanisms contribute to defective helper T (TH)-cell function early in the course of HIV-1 infection. Furthermore, two murine models appear relevant to the study of late HIV-1 infection and suggest a role for CD8+ T cells in the prevention of symptomatic AIDS.