Development of a C57BL/6-+/+ TCR transgenic mouse containing the rearranged TCR α- and β-chain specific for the Db + HY male Ag results in production of a nearly monoclonal population of early thymocytes expressing the Db + HY reactive TCR. These thymocytes are autoreactive in H-2Db male mice and undergo clonal deletion and down-regulation of CD8. To study the effect of the lpr gene on development of autoreactive T cells, these transgenic mice were backcrossed with C57BL/6-lpr/lpr mice. T cell populations in the thymus and spleen were analyzed by three-color flow cytometry for expression of CD4, CD8, and TCR. The thymus of TCR transgenic H-2(b/b) lpr/lpr male mice had an increase in percent and absolute number of CD8(dull) thymocytes compared to TCR transgenic H-2(b/b) +/+ male mice. However, there was not a complete defect in clonal deletion, because clonal deletion and down-regulation of CD8 was apparent in both +/+ and lpr/lpr H-2Db HY+ male mice compared to H-2Db HY- female mice. The phenotype of splenic T cells was almost identical in TCR transgenic +/+ and lpr/lpr males with about 50% CD4-CD8- T cells and 50% CD8+ T cells. However, there was a dramatic increase in the SMLR proliferative response of splenic T cells from TCR transgenic lpr/lpr males compared to TCR transgenic +/+ males. To determine the specificity of this response, spleen cells from TCR transgenic lpr/lpr and +/+ mice were cultured with irradiated H-2(b/b) and H-2(k/k) male and female spleen cells. T cells from TCR transgenic C57BL/6-lpr/lpr male mice had an increased proliferative response to H-2(b/b) male spleen cells compared to T cells from TCR transgenic C57BL/6-+/+ male mice, but both lpr/lpr and +/+ mice had a minimal response to irradiated H-2(b/b) female or H-2(k/k) male or female stimulator cells. The splenic T cells from TCR transgenic lpr/lpr mice also had an increased specific cytotoxic activity against H-2(b/b) male target cells compared to TCR transgenic +/+ mice. These results demonstrate that there is a defect in negative selection of self-reactive T cells in the thymus of lpr/lpr mice and a defect in induction or maintenance of clonal anergy of self-reactive T cells in the periphery of lpr/lpr mice.