Acute GVHD was induced in unirradiated (DA x LEW)F1 rats by the injection of parental DA lymphoid cells. Serum sequentially derived from such animals was found to contain increased levels of antibodies reactive with 70-kDa heat-shock protein (hsp70). Elevated levels of antibody to hsp70 were detected as early as 7 days and were maximal 14 days after the induction of GVHD. These increased levels of antibodies reactive with hsp70 paralleled the onset of the signs and symptoms associated with the development of GVHD. Increased anti-hsp70 reactivity in GVHD sera was associated statistically with increased levels of hsp70-reactive IgM, IgG2a, and IgG2b. In addition, GVHD sera reacted with a 70-kDa protein found within (DA x LEW)F1 lymphoid tissue. These results demonstrate that increased levels of antibodies to hsp70 parallel the onset and development of GVHD in an acute model and are the first to show that antibodies reactive with hsp70 are associated with an ongoing disease process.