A genetic polymorphism of CD45R expression was identified resulting in a lack of CD45R- lymphocytes in approximately 8% of healthy individuals. Family studies revealed an autosomal dominant mode of inheritance of the variant CD45R expression pattern. PBMC from donors possessing the variant type did not lose the CD45R antigen after in vitro activation, whereas a decrease of CD45R molecules was readily detected in individuals with the normal pattern. The expression of CD45RO antigens, as well as memory cell function, did not differ between both groups. These data show that activation and in vivo priming of T cells is not necessarily associated with a loss of CD45R antigen expression.