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 then 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 and 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 degrees C or + 4 degrees 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 used for HLA-DR typing either by PHA-CF or B-LCT.