Eight healthy men infected with human immunodeficiency virus, type 1 (HIV) and eight HIV seronegative age- and sex-matched controls exercised on a bicycle ergometer (75% of VO₂max, 1 h). The percentages of CD4 ⁺, CD4 ⁺ 45RA ⁺ , and CD4⁺45RO⁺ cells did not change, whereas the absolute number of CD4⁺ cells increased twofold during exercise and fell below prevalues 2 h after. The neutrophil count increase was more pronounced after exercise in the controls compared with in HIV-seropositive subjects. The percent CD16⁺ cells, and the natural killer (NK) and lymphokine activated killer (LAK) cell activity increased during exercise, but this increase was significantly less pronounced in the HIV-seropositive group. The results suggest that in response to physical stress, HIV-seropositive subjects have an impaired ability to mobilize neutrophils, NK and LAK cells to the blood. Furthermore, because the total number of CD4⁺ cells, but not the percentage of CD4⁺ cells, changed in response to exercise, this study further strengthens the idea that the percentage of CD4 ⁺ cells is preferable to the number of CD4 ⁺ cells in monitoring patients seropositive for HIV.