Abstract: A two site cross-reaction immunometric sandwich assay method for the detection and measurement of an analyte, such as creatine phospho-kinase-MB, in serum comprising the selection of two different antibodies each of which is specific to a different analyte but each of which will cross-react with the analyte of interest. The first antibody is reacted with the unknown sample utilizing a solid-phase to bind the first antibody. Separation of the solid and liquid portions of the first reaction is accomplished and the solid portion thereof is reacted with the second antibody which is tagged. The solid portion and liquid portion of the second reaction are separated and the solid portion is tested for the tag as an indication of the presence of said analyte. With particular reference to testiong for creatine phospho-kinase-MB in human serum, the cross-reacting antibodies utilized are antibody to creatine phospho-kinase-BB and creatine phospho-kinase-MM.