The expanded T-cell population of MRL/Mp-lpr2lpr mice is abnormal from a variety of standpoints. We have already shown that T-cell receptor expression and modulation are aberrant in the predominant CD4- CD8 (DN) T cell population. To investigate these abnormalities further, we examined CD3 expression and modulation in subpopulations of +/+ and lpr T cells and measured mitogen-induced Ca++ mobilization in DN lpr T cells. We found that expression and modulation of CD3 in CD4hi and CD8hi lpr single positive (SP) T cells are similar to that in +/+ T cells. We have, however, identified additional lpr cell subsets that are CD4lo or CD8lo. Their expression and modulation of CD3 are intermediate, between that of SP and DN lpr T cells. These subpopulations may thus represent a transitional stage between the SP and DN populations. The rapid modulation of CD3 in the DN population does not appear to be merely related to the lack of expression of CD4 or CD8, and may in fact cause (rather than result from) low CD3 expression. In addition, we observed impairment of CA++ mobilization in DN lpr T cells in response to concanavalin A or anti-CD3 antibody. These findings further define the abnormalities of T cells from lpr mice.