The outcome of bone marrow transplantation has been shown to be improved by matching of the HLA class I and class II antigens. As to the HLA class II, while there is a general agreement on the relevance of HLA-DR matching, the role of other HLA class II loci remains to be established. Among these, there is a growing interest for the possible role played by the HLA-DP locus because of its extensive polymorphism. Analysis of the correlation between DP compatibility and graft survival has been so far hindered by technical difficulties. In fact, DP allelic products cannot be easily typed serologically and, at the genomic level, the standard SSO technique foresees the use of 20 labelled oligoprobes. We have recently described a simple electrophoretic technique that, applied to HLA-DPB1 locus, is at least as discriminative as oligotyping, but it does not require probes. This method is based on the changes of the suprahelical structure of HLA molecules produced by mismatched base pairs in critical positions. These conformational changes in turn produce a degree of retardation in electrophoresis which can be allele-specific. We think that this technique, which has proven to be useful in the typing of several HLA loci, is particularly suitable for this kind of study where, often, a small number of samples at a time needs to be analyzed.