During the last year we were able to clarify the genetic aspect of the conversion from acute to hyperacute rejection. The conversion lacks genetic specificity in the sense that any disparity between donor and recipient may elicit it, as long as it is of sufficient strength; MLC disparity is effective because of its strength, not because of any specificity. The effector cells lack Lyt specificity. Lyt 1 as well as Lyt 2 and 3 cells will effect conversion. Experiments on T-cell-\ depleted and nude mice suggest that the transferred cells are the actual effectors of the conversion, and not merely inciters of a reaction by the recipient. The kinetics of effector cells have been studied thoroughly by means of adoptive transfer. Cells from lymph nodes draining antigenic sites become transfer effective after a time period dependent on the nature of the antigenic site: after intradermal sensitization it takes 48 hours; after skin grafting, 9 days. With either method transfer effectiveness ceases after 5 days, unless the cells are transferred to a nonsensitized syngeneic recipient. Intraperitoneal sensitization has singularly and consistently failed to elicit transfer-effective cells. There is evidence that tranfer effectiveness ceases when and because the cell donor reaches a high level of sensitization--a puzzling cause-and-effect relationship.