Screening for anti‐HLA‐DR sera was performed by complement fixation on PHA stimulated peripheral blood lymphocytes (PHA‐CF), or cultured B lymphoid cell lines. Out of 1,350 sera from multiparous women, multitransfused patients, and patients transfused during extra‐corporal circulation (ECC), 219 contained anti‐HLA‐DR activity (16.2%). Anti‐HLA‐DR antibodies developed after ECC were often high titered (1:10 to 1:100). In half of these sera anti‐HLA‐A, B antibodies were weak or absent, making it possible to use them as anti‐HLA‐DR reagents without platelet absorption. Of the 219 positive sera 51 contained defined anti‐DR antibodies (20 monospecific and 31 bi‐or multispecific). The 13 best sera recognized DR1 to DR7 specificities with r values from 0.83 to 1.Twenty‐four sera selected by CF were also studied by lymphocytotoxicity technique against peripheral blood B lymphocytes (B‐LCT). Both PHA‐CF and B‐LCT techniques gave similar results, detecting the same specificities and showing comparable sensitivity.The advantages of CF are: easy storage of target cells at −80°C or +4°C, and fast reading. For these reasons PHA‐CF or CF on cultured B lymphoid cell lines can be proposed for large scale screening of anti‐HLA‐DR sera. The sera thus screened can be use