To characterize the pre-transplant cytotoxic T cell precursor (CTLp) activity of unrelated bone marrow donors towards their HLA-ABDR compatible recipients, we have established T cell clones from the primary (positive) CTLp cultures. Two "perfectly matched" oligo-typed DR/DQ identical combinations and two A/B identical, DR subtype incompatible combinations have been studied in detail. Analysis of the clones shows that in the first two combinations the alloreactivity was directed towards a subtype of A2 (A205) and a subtype of Bw60 respectively. In the other two cases, where class II incompatibilities were present, we have not yet defined the exact specificity. However, because the majority of recipient specific clones were of the CD8 phenotype, it is very likely that also in these two cases differences in class I are being recognized.