Embryonic MPCs are capable of suppressing T-lymphocyte proliferation in vitro. To evaluate whether embryonic MPCs possessed immunoregulatory function, lymphocyte transformation test and mixed leukocyte reactions (MLRs) are carried out. In the presence of MPCs of BM, AoM and YS origin, in vitro proliferation of CD3⁺ T lymphocytes stimulated by nonspecific mitogens (A) and allogeneic lymphocytes (B) is significantly suppressed. The results represent the means (± SEM) from 3 independent experiments. Significance is determined using the Student t test: *P < .05; **P < .01, compared with the control data.