Summary1. Mice thymectomozid immediately after birth do not grow normally, fail to resist common infections, and die before the 4th month of life. 2. Thymectomy performed within 24 hours after birth allows the acceptance of skin homografts in all donor-host combinations studied, even in those differing at the H-2 histocompatibility locus, such as the C3H with (A X C3H)F1 and the DBA/2 with C3H. 3. Thymectomy at 35 days of age results in slight prolongation of skin homograft survival only in few animals in strain combinations sharing the H-2 locus.