BACKGROUND: The authors studied to estimate the frequency of irregular antibodies and their significance in blood transfusion and antenatal care in Korea. METHODS: Irregular antibodies were tested by immediate saline spin, 37degrees C albumin and antiglobulin test for 2,008 transfusion candidates and 1,047 pregnant women at Severance hospital using commercial screening and identification cells (Dade, U.S.A.). RESULTS: The irregular antibodies were detected in 38 (1.24%) of total 3,055 subjects (transfusion candidates: 0.9%, pregnant women: 1.91%) . In transfusion candidates, the detected antibodies were Lewis antibodies, cold antibodies (anti-M, anti-P 1), Rh antibodies and unspeified warm antibodies, and their distributions were 56%, 22%, 17%, and 5%, respectively. In pregnant women, the detected antibodies were Lewis antibodies, Rh antibodies, anti-Jra, and unspeified warm antibodies, and their distributions were 45%, 45%, 5% and 5%, respectively. At immediate saline phase, 58% of irregular antibodies were detected. At 37degrees C albumin phase, 90% of irregular antibodies were detected and only 10% of irregular antibodies were detected at antiglobulin phase. CONCLUSIONS: Although the prevalence rates of clinically important irregular antibodies were low, 1/1000 of irregular antibodies could not be detected. Therefore, irregular antibody screening should be performed in all pretransfusion test. And, if antibody detection tests are negative, immediate saline crossmaching methods are acceptable in Korea.