Accelerated skin graft rejection following transfer of cells sensitized to a 'weak' histocompatibility antigen(s)

C3H/HeJ mice were immunized with either CBA/J of C57L/J skin grafts and then tested for secondary responses against both strains. Secondary responses were measured by six day graft rejection, ability of syngeneic spleen cells to transfer skin graft immunity to sublethally irradiated mice, and graft versus host disease (GVHD) in lethally irradiated CBA/J mice receiving C3H/HeJ spleen cells. Although failure to transfer a primary response against 'weak' histocompatibility antigens with syngeneic spleen cells was noted, the secondary responses demonstrated a cross reacting antigen(s) which seems to be of greater immunologic strength than the postulated H 13 difference. Possible explanations for this finding include the existence of multiple undescribed loci with cross reacting alleles, or an increase in immunogenicity of 'weak' transplantation antigens when acting in concert with 'stronger' antigens.