Peripheral blood mononuclear cells were quantified for the subsets of CD4, CD8, and CD19 lymphocytes by using CD45RA (2H4), CD29(4B4), CD57, CD5, CD10, Leu8, HLA-DR, and TCR gamma delta-1 monoclonal antibodies and dual color immunofluorescence. A comparative analysis of lymphocyte subpopulations was made among 52 HIV-infected and 50 age-matched control children and 30 HIV-seropositive and 27 negative control adults. A significant decrease in the CD4+CD45RA+ "naive" cells was much more marked in HIV-infected children than in HIV-infected adults. A significant percentage increase in the CD4+CD29+ "memory" cells was observed in HIV-infected children but not in infected adults; however, the absolute numbers were usually decreased in all age groups. The mean percentage and absolute numbers of CD4+CD7+ and CD4+Leu8+ cells were decreased in HIV-infected children, although usually not significantly. The CD3+TCR gamma delta-1+ did not show any change in the infected children tested. The mean percentage and absolute number of the CD8+HLA-DR+ cells increased significantly in HIV-infected persons of all ages. The CD8+CD57+ cells were increased in percentage and absolute number in HIV-infected children ages 1-4 and 4-8 years. In the adults, no change was noted in either the percentage or absolute number of CD19+CD5+ B cells, a finding similar to that noted in HIV-infected children above 1 year of age. Although adults showed a significant decrease in both percentage and numbers of CD5- B cells, an increase was noted in the 7- to 12-month-old HIV-infected children. The CD19+CD10+ cells showed a slight but significant decrease in the youngest age group and a significant increase in the older age groups of HIV-infected children. These findings indicate that several lymphocyte subpopulations are altered differentially during HIV infection in children of varying ages and in adults.