Anti-I-region associated (Ia) antigen sera specificially supress the stimulation reaction in unidirectional mouse MLR. No effect on the response was observed with these antisera. In addition, anti-H-2 sera did not affect stimulation, however, in some cases diminished the response. The data indicate that Ia antigens and LAD might be identical. Stimulation in mixed lymphocyte culture reaction (MLR) or graft vs host reaction has been shown to be governed by genes located in the I-region (immune response region) of the H-2 complex in mice (Bach et al. 1972 and Klein and Park 1973) and it appears that the H-2K and H-2D regions play only a minor role in that reaction. Recently, antigens have been described (Hauptfeld et. al. 1973, Götze et al. 1973, David et al. 1973) which are controlled by the I-region, so-called I-region associated equals Ia antigens. If these antigens are related to determinants important for the MLR, then one might predict inhibition of the MLR. Abbasi et al (1974) reported that poly-specific anti-H-2 alloantisera inhibited the H-2 complex controlled stimulation in MLR. Using antisera produced in I-region congenic strains, we have been able to show complete and specific inhibition of the stimulation in MLR, without affection of the response, thus indicating that Ia antigens and lymphocyte activating determinants might be identical.