Intrauterine thymectomy was carried out on days 36, 40, 48 and 68 (neonatal) of foetal life to investigate the role of thymus and emigration of thymocytes in the development of mitogenic (PHA, Con A, DxS) responses in the guinea pig. The results provide further evidence that emigration of thymocytes is a continuous process which takes place already at the time of demarcation between thymic cortex and medulla (before day 36 of gestation) and again immediately after birth. In the lymph nodes of guinea pigs thymectomized on day 36 in utero a small PHA and Con A responding cell population exists at the time of birth but not any more 3-4 mth later. This population is considered to depend on the transplacental thymic product(s). However, spleen and blood lymphocytes of animals thymectomized on day 36 in utero respond to PHA and Con A to some extent at the age of 3-4 mth, suggesting that the population emigrating before day 36 has a longer survival in the spleen and blood than in lymph nodes. Thymectomy at 40 days of gestation leaves an additional population which can express a good PHA and Con A response even in lymph nodes without a direct thymic influence. Responses to DxS were not significantly affected by the intrauterine thymectomy. Thus, intrauterine thymectomy at 36 days of gestation induces a severe but not complete T cell depletion in the guinea pig.