Viable mouse skin flaps conjugated with poly‐DL‐alanine chains were obtained byin vitrotreatment with the N‐carboxyanhydride of DL‐alanine. It was assumed that the reacting groups are directly and covalently linked to the histocompatibility antigen molecules, or are in their close vicinity. Therefore, the transplantation of the poly‐DL‐alanylated skin flaps onto syngeneic recipients might provide a model for studying the behavior of antibodies of poly‐DL‐alanyl specificity in graft rejection.When purified rabbit anti‐poly‐DL‐alanyl antibodies were passively administered to mice transplanted with poly‐DL‐alanylated skin, the graft survival was sharply reduced as compared with that of the controls. This observation might suggest that certain antibodies which have been produced in rabbits, can reduce graft survival when passively administered to mice. If mice were immunized with poly‐DL‐alanyl‐RSA before grafting, in order to elicit an active production of anti‐poly‐DL‐alanyl antibodies, the poly‐DL‐alanylated skin graft survival was similar to that of the controls. It is possible that this phenomenon may be caused by