Phenotypic features and proliferative ability of thymocytes, splenocytes and peripheral blood lymphocytes of 20-22 weeks human fetuses affected by Down's syndrome (DS) were studied and compared to those of fetuses of the same gestational age with a normal karyotype. In the thymus of both DS and normal fetuses, the great majority of cells was CD1+, CD2+, CD5+, CD4+, CD8+; using double fluorescence analysis, these markers could be detected on the same cell. About 50-60% showed CD3 antigen and about 40-50% presented the alpha beta T cell receptor. Thymocytes with NK markers (CD16, CD57, CD56) were not found. After stimulation with phytohemagglutinin, thymocytes showed a low but detectable proliferative capability, while splenocytes and peripheral blood lymphocytes showed a high responsiveness to the mitogen. These data show that the impaired immune system in DS is not associated with gross abnormalities of phenotypic T cell development in the fetal thymus or with an inability of such fetal cells to proliferate after a mitogenic stimulus.