The chance of finding an unrelated bone marrow donor depends on the size of the donor registry, the frequency of the patient's HLA phenotype and the degree of histoincompatibility considered acceptable. We have studied 60 unrelated bone marrow donor searches and the probability of finding a donor when increasing HLA-compatibility requirements were applied. For 21 (35%) of the patients no donor could be identified who was (serologically) matched for all six ABDR antigens. For the remaining 39 (65%), a median of two (range 1-20) blood samples from serologically-matched potential donors was obtained for further analysis. Eighteen (30%) patients appeared to be mismatched with all the pre-selected donors because of DR (sub-)type mismatches detected by oligotyping. A further 7 (12%) patients did not have an MLC and/or cytotoxic T lymphocyte precursor (CTLp) negative donor. Thus, a 'perfectly matched' donor was found for 11 (18%) patients. To find a suitable donor for more patients we propose that many or all the potential donors identified in the various registries for a particular patient are simultaneously evaluated as many of the serologically ABDR-matched donors will appear to be incompatible after high resolution HLA-typing. This strategy will not only allow selection of the best matched donor more rapidly but the consequent use of high resolution typing might also lead to the definition of a certain number of acceptable mismatches.