The distributions of lymphocyte subsets and monocytes in the peripheral blood mononuclear leukocytes of 72 normal children from 2 months to 13 5/12 yr were examined using quantitative immunofluorescence analysis with monoclonal antibodies. Distinct decreases with age were found in the total leukocyte counts, the percentages and the absolute numbers of peripheral blood mononuclear leukocytes. The percentages of Leu-2a+ cells, Leu-7+ cells, and Leu-M3+ cells significantly increased with age, whereas the percentages of Leu-3a+ cells, Leu-4+ cells, and 2H7+ cells significantly decreased with age. As a result, ratios of Leu-3a+/Leu-2a+ decreased with age. No prominent differences with age were found in the proportions of Leu-10+ cells and HLA-DR+ cells.