Serum and Mg2+-ethyleneglycoltetraacetic acid (EGTA) plasma from 14 domestic or laboratory animal species had no anti-complementary activity for sheep complement (C). Serum samples from 8 species and all 14 Mg2+-EGTA plasma samples were anti-complementary towards guinea-pig C. Thus, use of sheep C in a warm complement fixation text (with human erythrocytes sensitized with sheep antiserum, IgM, slow or fast γ-globulin antibody), in the presence or absence of Mg2+-EGTA, may be a satisfactory method of eliminating anticomplementary activity.