BACKGROUND: Anti-HLA antibodies are most frequently induced by transfusion or pregnancy, and these anibodies can be used as antisera for HLA typing. However these antibodies may elicit adverse reactions such as transfusion reaction or rejection of transplanted organs. In this study, frequency and specificities of antibodies against HLA class I antigens were determined in multiparous Korean women. METHODS: Sera from 671 multiparous women were tested for anti-HLA antibody screening by standard microlymphocytotoxicity test using 49~50 lymphocyte panels. PRA(panel reactive antibody) values were calculated as percentage of postive panels among total lymphocyte panels tested. HLA antibody specificities and reaction strengths were determined by analysis of serologic reaction patterns. RESULTS: A total of 671 sera were tested and 124 sera(18.5%) were positive for HLA antibodies. Among HLA antibody-positive sera(n=124), 117(94.4%) showed PRA values of < OR = 50% and only 7(5.6%) showed PRA values of >50%. Specificities of HLA antibodies were identified in 51 sera(41.1%) and 18 sera(14.5%) contained reagent quality antibodies(r> or =0.8, SI> or =70%), corresponding to 2.7% of total multiparous women. Among these, 4 sera had monospecific HLA antibodies and 14 sera had HLA antibodies against two or more antigens: 4 sera containing HLA antibodies against 7 CREG(cross reactive group), 5 sera containing antibodies against 5 CREG. CONCLUSION: Through the analysis of frequency and specificity of HLA antibodies in 671 multiparous women, it is concluded that HLA antisera can be obtained from multiparous women as effectively as from pregnant women. The frequency of high level of sensitization(PRA>50%), which can elicit problems in relation to transfusion or organ transplantation, is very low(1.0%).