Transplantation immunity was examined in mouse H-2− and non-H-2-disparate strain combinations by the macrophage electrophoretic mobility (MEM) assay. Lymph node cells from the recipients rejecting histoincompatible skin grafts were incubated in the presence of solubilized histocompatibility antigens prepared from spleens of the graft donors. Product(s) of lymphocyte-antigen interaction in appropriate combinations (sensitized lymphocytes with the sensitizing antigen) significantly diminished the electrophoretic mobility of guinea pig macrophages in all of the histoincompatible strain combinations examined. The data indicate that transplantation immunity in mice can be detected by the MEM assay.