A study was made of PHA-stimulated thymic lymphocyte transformation in 10 human fetuses ranging from 12 to 22 weeks of gestation. The results were compared with those obtained using circulating blood lymphocytes of the corresponding fetuses in seven cases. The degree of transformation of blood lymphocytes is independent of fetal age. On the other hand, thymic lymphocyte transformation is age-related. Until the 18th week of gestation, the per cent of transformed thymic lymphocytes is comparable to that of blood lymphocytes. After the 18th week of gestation, the per cent of transformed thymic lymphocytes diminishes to the level of adult thymic lymphocytes. The significance of these findings is discussed.