Summary The effect of thymectomy performed in mice either at birth or at 30 days of age on survival time of homologous skin grafts was studied. The results demonstrate that in certain donor-host strain combinations isogenic at the H-2 locus such as DBA/2 with (Balb/C X DBA/2)F1 and Z(C3H) with Ce, previous thymectomy of the host resulted in prolonged survival of the skin homografts. However, in other strain combinations differing at the H-2 locus such as Z(C3H) with (A X Z)F1 hybrid, thymectomy of the recipient resulted in a slight and perhaps unsignificant prolongation of skin homograft survival.