Transfer of bone marrow (BM) from autoimmunity-prone mice homozygous for the lymphoproliferation (lpr) mutation to irradiated congenic+ I+ recipients has previously been shown to result in a syndrome similar to chronic graft-vs.-host (GVH) disease. It has been suggested that this syndrome may be due to an antigenic difference caused by the lpr mutation itself or to antigenic differences at loci closely linked to the lpr locus (Theofilopoulos, AN et al., J. Exp. Med. 1985.262: 1; Mosbach-Ozmen, L. and Loor, F., Thymus 1987.9: 197). However, the results presented here indicate that alloantigenic differences do not play a role in this syndrome. Instead, the chronic disease observed in Iprllpr-++/+ BM chimeras appears to develop as a result of a functional defect associated with the lpr mutation which is expressed shortly after transfer of lprllpr BM to irradiated recipients. This defect causes an increase in the levels of serum IgGl and IgG2, which peak at 4-5 weeks post-transfer and then decline to normal levels by 9-10 weeks post-transfer. Inflammation similar to that observed in classic GVH reactions accompanies excess IgG production in congenic+ I+ recipients but not in lprllpr recipients of Iprllpr BM. We demonstrate that the GVH-like response occurring in lprllpr++ I+ chimeras is dependent on mature T cells, but that either lprllpr or+/+ Tcells can support this reaction. These results suggest that transfer of lprllpr BM to normal mice causes immunoregulatory disturbances which lead to nonspecific activation of Tcells. We speculate that lprllpr BM causes a GVH-like reaction in+/+ recipients but a systemic lupus erythematosus-like syndrome in lprllpr recipients because of intrinsic differences in the+ I+ and lprllpr host environments. Considering these findings, the lprllpr++/+ GVH model may be useful for analysis of factors capable of inducing undesirable reactions in clinical BM transplantation between nominally histocompatible donors and recipients, in addition to being informative about the nature of the lpr mutation itself.