AbstractThe nature of Ia antigens inducing lymphocyte proliferation and immune interferon (IFN‐γ) production in primary and secondary murine mixed lymphocyte cultures was studied by means of monoclonal anti‐Ia antibodies. In primary mixed lymphocyte cultures, both lymphocyte proliferation and IFN‐γ production were found to be controlled by Ia determinants coded for by the I‐A subregion. E alloantigen‐recognizing antibodies displayed consistent stimulation of both lymphocyte functions, instead of inhibition, suggesting a regulatory mechanism of the E molecule on lymphocyte activation. In secondary mixed lymphocyte cultures fewer Ia determinants seemed to be involved in lymphocyte proliferation, since only antibodies directed against specificities Ia.17 and Ia.19 were found capable of blocking lymphocyte proliferation. In contrast, IFN‐γ production, although significantly decreased by antibodies against specificities Ia.17 and Ia.19, was not completely abrogated, even by a mixture of all anti‐Ia antibodies.The data indicate that (a) in primary mixed lymphocyte reaction lymphocyte proliferation and IFN‐γ production are triggered by the same Ia specificities coded by the I‐A subregion and (b) in secondary mixed lymphocyte reaction, whereas lymphocyte proliferation is restricted to fewer determinants, IFN‐γ production shows a lower threshold of activation, being induced also by determinants ‘silent’ in prima