We concur fully with the comments on interpretation of CK-MB isoenzymes in Dr Dubin's communication: (1) The CK-MB isoenzyme fraction may be positive in the qualitative assay system from extensive skeletal injury alone. (2) Appropriate dilution of total CK activity before isoenzyme separation will render the CK-MB band negative in the majority of cases because CK-MB is diluted below the level of sensitivity in the assay system. (3) Persistence of the CK-MB band after dilution is suggestive of concurrent myocardial injury, as it represents a greater percentage of CK activity in the MB fraction. We agree that a negative CK-MB band after dilution does not exclude myocardial injury, as extensive skeletal damage may mask the effect of lesser myocardial involvement. This point deserves the emphasis that he brings to it.