The reactivity of selected serum reagents used in identifying certain bovine erythrocytic antigens was studied quantitatively by varying the concentrations of antibody and complement independently. The serum reagents examined in this way ranged from those which act rapidly to those which failed to produce complete lysis even after extended incubation. The presence of an inhibitor, possibly a nonhemolytic antibody, is postulated to explain the more slowly acting serum reagents. A comparison of rabbit and guinea pig serums as sources of complement suggests that compatibility is a matter of unit specificities rather than of species specificity.