A comparison of the HLA class I typing in 50 unrelated individuals by means of serological and molecular genetic (PCR-SSP) methods was carried out. DNA-typing is more fast and reliable method in comparison with serology. It is necessary to introduce molecular genetic methods for the detection of HLA class I alleles. On the other hand there are alleles, which are not expressed on cell surface. In our laboratory both methods are established and the results of both were compared. It may be useful for determining the selection strategy of HLA-identical donor-recipient pair suitable for bone marrow transplantation. The results demonstrated 9% misassignments of HLA-A antigens by serology, 11% of HLA-B and 39% of HLA-C. The serological discrepancies found were of three categories: false negatives, false positives, and an incomplete typing. The vast majority of the discrepancies were due to a combination of relatively low expression of HLA antigens, lack of serological reagents and misclassification of antigens within cross-reactive groups. These results indicate that nowadays the serological typing is insufficient for clinical histocompatibility testing. (Tab. 3, Ref. 16.)