SUMMARY A mouse alloantiserum, raised by immunizing B10.D2/n mice with B10.M spleen, has been found to retain cytotoxic activity against a proportion of lymph node cells even after exhaustive absorption with B10.M red blood cells. Titration and absorption with a number of strains showed that the absorbed serum retained antibodies against two new specificities. The absorbed serum gave significant enhancement of B10.M skin grafts on B10.D2 and reacted mainly with B cells.