Fractionation of the creatine kinase-MB isoforms is promising for use in diagnosing acute myocardial infarction and for monitoring myocardial perfusion status after thrombolytic therapy. An immunochemical reagent intended for use in fractionating the MB1 and MB2 isoforms of creatine kinase-MB was examined before and after immunoextraction, qualitatively by visually examining electrophoresis separation of various MB1 and MB2 mixtures, and quantitatively by comparing the observed and predicted enzymatic activity of various MB1 and MB2 mixtures. Qualitatively the reagent showed greater reactivity for MB1 than for MB2, as demonstrated by a marked decrease in the MB1 electrophoretic region following immunoextraction. Quantitatively, the reagent consistently eliminated about 75% of MB1 activity; however, the assay also eliminated about 40% of MB2 activity from isoform mixtures. Although the performance of the immunochemical reagent was not ideal, the greater reactivity for MB1 may have clinical use.