Hematologic values and lymphocyte subpopulations were determined in normal fetal blood during the second trimester of gestation. In these samples the platelet, erythrocyte, and leukocyte counts were significantly lower than in adults. Large red blood cells with a high hemoglobin content were present. Before the twentieth week of gestation, erythroblasts made up about half of the nucleated elements. Lymphocytes formed most of the leukocytes, and their absolute numbers were comparable to those in adults. Most of the fetal blood lymphocytes expressed T- or B-cell surface differentiation antigens. The percentage of T-cells was lower and that of B-cells was higher than in the adult. A high OKT4/OKT8 ratio was present. It was due to a low percentage of OKT8-positive cells. Lymphocytes with a natural killer cell phenotype were rare. Most lymphocytes were OKT10 positive, but almost none reacted with the antithymocyte antibody OKT6. These results give additional information about the development of blood cells in early human life. They can be used as reference values for the prenatal diagnosis of hereditary or acquired anomalies of the hematologic and immunologic systems.