Abstract: In this paper.evidence in vivo that Langerhans' cells play as important role in immunological rejection is presented. The rejection of H-Y incompatible skin grafts in BN/Ss rats seems related to the Langerhans cell density of the graft per se. The more Langerhaas cells, the higher possibility of being rejected. Survival of these grafts can be obviously prolonged by replacing their Langerhaas cells with aliogeneic but not with spageneic female cells.