Sheep red blood cells can surround, in vitro, some human peripheral blood lymphocytes in a formation called a rosette. The number of rosetteforming cells (RFC) in 50 normal persons had a wide range (4-40%). The organs of 13 human fetuses (11-19 wk conceptional age) were examined for the presence of RFC. The thymus possessed the highest percentage of RFC, the maximum being 65% of total thymocytes in two 15-16 wk fetal specimens. Blood RFC were always present and their number slightly increased in the oldest fetuses. The bone-marrow showed 0-8% in the six fetuses studied. RFC were found in the spleen around the 13th wk and in the liver around the 17th wk of gestation. These observations lead to the hypothesis that human blood RFC may be chiefly thymic derived. Studies of patients with immunological disorders support this hypothesis: one patient with Nezelof syndrome had no blood RFC and four patients with Wiskott-Aldrich syndrome had a low number of blood RFC (1 and 1.5%). Patients with acquired hypogammaglobulinemia showed a normal percentage of RFC. With the fetal thymocytes, the percentage of inhibition with anti-mu serum increased with the fetal age to become complete in the oldest fetuses studied. Incubation of the oldest fetal thymocytes or the blood lymphocytes with anti-gamma serum of anti-mu serum completely inhibited the rosette formation. These results suggest that mu-chain determinants are present on human fetal thymocytes and blood RFC. The significance of the presence of gamma-chain determinants on these cells is unclear.