Abstract Previous observations from our lab and many others have suggested that MLR determinants are composed of I region gene products. We extended our original observations by studying the reactivities of responder cells obtained after in vitro primary stimulation (primed responder cells = PRC) with selected stimulator cells. These studies suggested that MLR determinants exist as “interaction” products of several I region genes which can act in cis- or trans- position. Furthermore, these studies suggested the existence of specific clones of PRC which could recognize unique F1 MLR determinants. Using serially restimulated PRC which had been in culture for over three months, we derived colonies of alloreactive cells (using soft agar cloning techniques) which have remarkably restricted reactivities. Studies with these “cloned” PRC supported our original premise concerning clones of unique F1 reactive PRC and suggested the existence of unique “homozygous” MLR determinants and clones of cells which specifically recognize such determinants.