Human fetal lymphoid cells from the thymus, spleen and peripheral blood of 25 fetuses (14-25 weeks of fetal age) were studied for their ability to respond to phytohemagglutinin(PHA), pokeweed mitogen(PWM), and streptolysin-0(SLO). Lymphocyte transformation by PHA and PWM was observed in the thymus, spleen and peripheral blood at the 14-16th week of fetal age. The percentage of transformed thymic lymphocytes, observed 3days after inoculation, decreased as the age of fetuses increased. On the other hand, splenic lymphocyte transformation increased according to age. The percentage of thymic lymphocytes transformed by PHA, observed 3 days after inoculation, was comparable to that of transformed splenic and peripheral blood lymphocytes. However, a lower percentage of transformed lymphocytes in the thymus was observed 6 days after inoculation compared to that of transformed splenic lymphocytes. In the thymus, the response to PHA observed 3 days after inoculation was higher in ratio than that observed 6 days after. These results suggest that thymic lymphocytes consist of two subpopulations which exhibit "maximum response" 3 or 6days after inoculation. The percentages of thymic lymphocytes and splenic lymphocytes transformed by PWM were the same. In PWM-stimulated transformation, no difference was detectable between these two organs. There was no difference in transformation whether observed 3 or 6days after inoculation with PWM. Lymphocyte transformation to SLO was observed in the thymus at the 17th week and in the spleen at the 19th week. Although cord blood lymphocytes were not exposed to streptococcal extracts, they were transformed by SLO. These results suggested that SLO was a nonspecific mitogen. Human fetal lymphoid cells from the thymus and the spleen of 9 fetuses (14-25 weeks) were examined for the presence of spontaneous rosette-forming cells (RFC) with sheep red blood cells. RFC were found in the thymus and spleen at the 14th week. The average percentage of RFC was 78.1% in the thymus and 12.1% in the spleen.