The action of fetal calf serum and gamma globulin-free calf serum in the rosette formation between sheep red blood cells and mononuclear cells was studied in the lymphoid tissues of 12 human fetuses and the blood of 50 adults. It was found that serum increases the number of rosette-forming cells (RFC) in the fetal thymus, spleen, and blood, and in the adult blood. In contrast, rosette formation was decreased or abolished in fetal bone marrow. In one fetus studied, the bone marrow RFC showed phagocytic activity while in the other lymphoid tissues the RFC did not possess such activity. RFC from adult blood were not able to ingest carbon particles. Some normal subjects were followed during several months for their blood RFC performed in presence of serum; no important variation was noted. Therefore, the use of serum appears to be a valuable tool for studying thymus-derived cells.