Hemopoietic cells have been reciprocally transferred between two lines of mice (MRL lpr/lpr and MRL +/+) that are congenic, differing only at the lpr (lymphoproliferation) and possibly closely linked genes. The lpr strain develops a significantly more severe and fast-paced lupus-like syndrome than +/+ strain, along with a substantially larger lymphoid mass. The results showed that: (a) hemopoietic cells of such mice were sufficient to induce the respective disease phenotypes in lethally irradiated syngeneic recipients; (b) cells of MRL +/+ mice maturing in an MRL lpr/lpr environment essentially retained the disease-producing characteristics of the donor, i.e., they induced late-life lupus without lymphadenopathy; but (c) MRL lpr/lpr cells transferred into irradiated MRL +/+ recipients unexpectedly failed to induce the early-life severe lupus and lymphoid hyperplasia of the donor, instead they caused a severe wasting syndrome resembling, in many respects, graft-vs.-host disease (GVHD). This GVHD-like syndrome developed after transfer of MRL lpr/lpr fetal liver, bone marrow, or spleen cells, and was not abrogated by elimination of T cells from the inocula. Thymectomy of the MRL +/+ recipients retarded, but did not prevent, the wasting disease. The unidirectional nature of this disease suggests that the lpr mutation conferred either a structural or regulatory defect that interfered, blocked, or altered the expression or structure of certain lymphocyte antigen(s). As a result, the MRL +/+ cells that did express this antigen(s) were recognized as foreign, and stimulated a graft-vs.-host reaction. These findings may allow definition of a new kind of rejection phenomenon caused by non-H-2 products, and may extend our understanding of the means by which the lpr gene adversely affects lymphocyte regulation and homeostasis.