The determination of lymphocyte subsets utilizing monoclonal antibodies and flow cytometry has become essential in the evaluation of immunological status. Using a standardized method it was found that in healthy children the percentage of CD 8+ (Leu 2+) positive cells increases significantly (P less than 0.01) during infancy, whereas the percentage of CD 4+ (Leu 3+) positive cells decreases with age (P less than 0.01). The percentage of CD 3+ (Leu 4+) cells remains constant. The ratio of CD 4/CD 8 positive cells is significantly (P less than 0.001) higher in infants than in older children. Other subpopulations (HLA DR+, Leu 7) were found to be constant in all age groups. For the comparison of data on lymphocyte subsets obtained by flow cytometry a standardized test procedure is important.