For the in vitro studying allograft immunity of mice the soluble factors of sensitized lymphoid cells were investigated by indirect macrophage migration inhibition test. As a result it has been shown that the sensitized lymphoid cells obtained from A (H-2(a)) mouse previously sensitized by a skin graft from C3H (H-2(k)) mouse elaborate macrophage migration inhibition factor (MIF) over a long period of time though migration inhibitory activity is not so strong. The sensitized lymphoid cells obtained from C3H mouse previously sensitized by a skin graft from A mouse show a strong migration inhibitory activity but the period of MIF production is short. In either instance MIF becomes measurable 5 days after skin grafting, and it can no longer be measured after the time when all the skin graft has become necrotic. In other words, the extent of the migration inhibition activity seems to reflect the degree of difference in the H-2 histocompatibility antigens, and the time-lapse changes in MIF would serve as a useful parameter of allograft rejection.