We have generated several transgenic mouse strains carrying a human immunodeficiency virus 1 (HIV-1) NEF/3' long terminal repeat (LTR) transgene under control of a T cell-specific promoter-enhancer element, showing a depletion of CD4+ T cells in the thymus and periphery. The immunological functions of the line with the most dramatic changes in lymphocyte populations, B6/338L, were analyzed in greater detail. The presence of the transgene in the heterozygous animal is associated with a dominant severe immunodeficiency. Older animals develop lymph-adenopathy and splenomegaly. CD4+CD8+ and CD4+CD8- single positive thymocytes already are depleted in these mice at the earliest stages in ontogeny, and peripheral T cells are reduced in frequency and present cell surface marker expression, which is characteristic for memory and activated T cells. The immunological response of B6/338L mice to several viral infections is also greatly impaired. Thus, the HIV-1 NEF/3' LTR as transgene in T cells can cause immunodeficiency and disease with striking similarities to a known retrovirus-induced immunodeficiency called murine AIDS (H. C. Morse III, S. K. Chattopadhyay, M. Makino, T. N. Frederickson, A. W. Hügin, and J. W. Hartley. 1992. AIDS. 6:607).