Lymphocytes from children with congenital thymic dysplasia were stimulated withmitomycin-C treated lymphocytes in the one-way stimulation test and with phytohemagglutinin (PHA). Cells of a patient with the sex-linked form of congenital thymic dysplasia consistently responded to allogeneic lyphocytes although the response to PHA was usually negative. Lymphocytes of two other patients did not respond to any form of stimulation; conversion of the PHA response took place in one of these children after transfusion with maternal and fetal lymphoid cells and was followed by a fatal graft-versus-host disease. Plasma from these children supported the growth of normal lymphocytes adequately. By our methods the carrier state could not be detected in patient's relatives. There is suggestive evidence that lymphocyte response, as demonstrated by these in vitro tests, depends upon the integrity of the thymus. Failure to respond to PHA in the presence of a response to allogeneic cells suggests functional heterogeneity within the lymphoid system. Allogeneic stimulation may be more useful than PHA in the study of patients with congenital thymic dysplasia.