Two definite acute hemolytic transfusion reactions occurred in a patient with chronic myelogenous leukemia in blastic crisis. Both suspect units were entirely compatible by routine crossmatch using commercial antiblobulin sera. However, both units were clearly incompatible using our own specific anti-C3 antiserum in the antiglobulin reaction phase of the crossmatch. Subsequently it was possible to predict in vivo compatibility using our anti-C3 antiserum in vitro. This case adds new evidence for the inadequacy of anticomplement activity in commercial antisera.