Spleen cells from mice infected with Friend leukemia virus (FLV) inoculated by the intravenous route give rise to macroscopically visible colonies in the spleens of normal F1 histocompatible hybrid hosts. A study of H-2 antigens as generic markers for identification of strains of origin of cells constituting the spleen colonies was undertaken. The standard cytotoxic test was demonstrated to be suitable for characterizing the H-2 antigens present on the surface of spleen cells from normal of FLV-leukemic parents of F1 hybrid mice. Individual colonies dissected out of the spleen of (C3HxC57B6/6) F1 recipients (H-2k/H-2b), 10 days after the intravenous graft of FLV-infected spleen cells of C3H origin (H-2k), were all sensitive to anti-C57BL/6 antibodies. In the same way, colonies obtained from the spleens of (DBA/2xC57BL/10) F1 recipients (H-2d/H-2b) grafted with DBA/2 leukemic spleen cells (H-2d) were all sensitive to both anti-H-2b and anti-H-2d antibodies. These results directly prove that the main cell population constituing a spleen colony arises from the recipient. The authors conclude that the spleen colonies do not result from the neoplastic proliferation of injected donor cells but rather from the multiplication of host cells transformed by Friend virus produced by the grafted cells.