Irregular antibodies are antibodies that are not regularly resent in the serum of a particular blood groups, e.g., anti-A1 in the A2B group or Rh antibodies in Rh negative individuals. Some of these antibodies are produced asa result of immunization by pregnancy or previous transfusion while others are developed although the patients concerned have never been exposed to foreign red cells. The presence of irregular antibodies is usually detected in discrepancies in ABO cell and serum typings, incompatible results in cross matching, or positive results in deliberate antibody screening tests. To understand the frequency of irregular antibody bamong Korean, screening tests were done with 2,518 serum samples of donor and 595 serum samples of pregnant and multiparous women which were collected during the period of Jan. 1980 to Sept. 1981. The results obtained are as follows; 1) With the screening of 2,518 donor sera, no irregular antibodies were detected by saline, albumin, and antiglobulin techniques, but one out of 2,518 cases(0.04%) was detected to be positive by papain test. But the antibody could not be identified because the serum specimen was insufficient. 2) With the screening of 595 pregnant and multiparous women's sera, one case of anti-E in group A, CDe/CDe pregnant, one case of anti-A1 in a cis A2B mother, and, one case of anti-P1 in 42 year old multipara were detected and identified with the use of pooled O cells, known Rh positive screening cell, Selectogen I, II and Resolve Panel A of Ortho Diagnostics.