The presence of the homozygous lpr gene (lpr/lpr) in MRL mice has been regarded as mandatory for the development of the early onset of lupus disease, T-cell dysfunction, and polyclonal B-cell activation. Congeneic MRL mice lacking the lpr gene (MRL +/+) display neither the lupus disease nor the immunological abnormalities within the first year of life. In this study we examined the cellular functions of MRL mice heterozygous at the lpr locus. The results indicate that MRL mice heterozygous at the lpr locus display intermediate delayed-type hypersensitivity compared with homozygous lpr/lpr mice on the one hand and congeneic +/+ mice on the other. Furthermore, proliferative responses to concanavalin A, measured by uptake of [3H]thymidine, were significantly lower in MRL mice heterozygous at the lpr locus than in +/+ mice, but significantly higher than in homozygous MRL lpr/lpr mice. Polyclonal B-cell activation, assessed by measurement of frequencies of IgG-secreting spleen cells, a prominent feature in MRL lpr/lpr mice, was significantly lower in lpr/+ mice and totally absent in +/+ mice. Furthermore, spleen cells spontaneously secreting auto-antibodies were found in large numbers in MRL lpr/lpr mice and in considerably lower but still significant numbers in heterozygous MRL +/lpr mice. In contrast, spleen cells from matched MRL +/+ mice did not display any spontaneous autoantibody production. Taken together, these data provide evidence for immunomodulatory properties of the heterozygous lpr gene.