[Stimulating determinants of the secondary mixed lymphocyte reaction (MLR-II)].

An anti-Ia immune serum (A . TL anti-A . TH) directed to the antigens of the stimulating cell, blocked the MLR-II. A significant correlation was observed between the anti-Ia reactivity of the immune serum, studied against a panel of eleven original H-2 haplotypes and the reactivity against the same panel of the in vitro primed responding cells (A . TL anti-A . TH). These results confirm the hypothesis that Ia antigens are the structures stimulating the MLR-II.