We have analysed two Caucasian families in which recombinant individuals have been identified. In both families, initial low resolution typing of class I and II antigens of possible patients referred for bone marrow transplantation and their respective potential donors (based on inherited haplotypes analysis) revealed them to be HLA identical and supposedly inheriting-non-recombinant haplotypes. The mothers were found to be DRB1*04 generic allele homozygous, but possessing two DRB1*04 specific alleles, DRB1*0403 and DRB1*0404 (family A) and DRB1*0401 and DRB1*0402 (family B). In both cases the patients inherited a maternal haplotype that is the result of a recombination event between the mother's HLA-DRB1 and -B genes on their chromosomes. Based on linkage disequilibrium it is likely that the recombinant haplotypes are present in the patients rather than their brothers. In both families, the results of the MLC in terms of relative response was positive. Thus, these cases illustrate the importance of high resolution DNA class II typing when assignment of MHC antigens is of extreme importance (i.e. bone marrow transplantation).