The B10.D2 cytolytic T lymphocyte (CTL) receptor repertoire specific for the H-2Kb alloantigen has been studied by determining the reactivity patterns of monoclonal CTL against a panel of seven different H-2Kb mutants. The repertoire is extremely diverse and contains a minimum of approximately 50 different specificities against unique antigenic determinants on the H-2Kb molecule. Each specificity appears at a maximum frequency of 1 in 18,000 CTL precursors. These studies have also served to dissect the antigenic composition of the H-2Kb molecule. Very few CTL clonotypes share recognition of all the mutants, thereby indicating the lack of conservation of a b-type antigenic region. In addition, the degree to which each mutant shares antigenic determinants with the standard H-2Kb molecule has been determined.