Since the earliest days of the HIV epidemic, the number of CD4⁺ T cells per unit volume of blood has been recognized as a major prognostic factor for the development of AIDS in persons with HIV infection. It has also been generally accepted that approximately 2% of total body lymphocytes circulate in the blood. In the present study, we have used a nondepleting humanized anti-CD4 monoclonal antibody labeled with the gamma emitter indium-111 to visualize the CD4⁺ T-cell pool in vivo in nonhuman primates with simian HIV infection. A strong correlation was noted between radiotracer uptake in spleen, tonsil, axillary lymph nodes, and peripheral blood CD4 T-cell counts (ρ = 0.75, 0.93, and 0.85, respectively, P < .005). The relationship between radiotracer retention in lymphoid tissues and CD4⁺ T-cell counts in the circulation was governed by an exponential law. These data provide an estimate for the total number of lymphocytes in the body as being between 1.9 and 2.9 × 10¹² and suggest that the partition between peripheral blood and lymphoid tissue is between 0.3% and 0.5%.