Acute graft versus host disease (GVH) can be induced in unirradiated adult (B6 X DBA/2)F1 by the inoculation IV of 60-100 X 10 to the six B6 spleen cells. GVH is not seen however, in F1 recipients of as many as 200 X 10 to the six DBA/2 spleen cells. This resistance to acute GVH is abrogated by irradiation of the recipient prior to donor cell inoculation suggesting that the recipient participates in this apparent resistance to GVH. Genetic studies have revealed that this resistance phennomenon is not H-2 haplotype specific but instead is influenced by multiple dominant non-H-2 genes of the donor. Inoculation of small numbers (20-30 X 10 to the six) B6 or larger numbers (60-100 X 10 to the six) DBA/2 donor cells into F1 recipients was found to induce the production by the recipients of antibodies to a wide variety of NuLV. These antibodies react with viral envelope-like determinants expressed by lymphoid and hematopoietic cells of the F1 and both parental strains. The possible role of these antibodies in the genesis of GVH is being studied.