reactivity does not seem to occur as rapidly. and one would expect to be able to continue to measure immune reactivity at times when enzyme activity is reduced or even possibly absent. Thus, since this radioimmunoassay is more sensitive than standard elecuophotometric and fluoromemc measurements of enzyme activity, and since measurements of enzyme activity may in some circumstances be misleading, we do not agree with D r s Moore and Norris that "measurement of enzyme activity should take preeminence, especially in such heterogeneous systems as the biological fluids"; in fact, we believe that a radioimmunoassay measurement of the isoenzymes is both quantitative and more accurate. This same reasoning has been applied to the quantificarion of many hormones and other substances in biological fluids.
Regarding the m o u n t of CK-B isoenzyme chat can be detected with this radioimmunoassay, we do state in the legend to Figure on page 53 [l] that "as little as 0.2 ng of CK-BB isoenzyme CM be recognized using this technique." The data for this statement are shown in Figure , where it is apparent that the addition of as little as 0.2 ng of unlabeled CK-BB isoenzyme results in a reduction in the amount of antibody bound to labeled CK-BB isoenzyme. This is tested in an assay volume of 0.6 ml. The assay is capable of detecting as little as 1 ng of CK-B per milliliter.
We are not certain of the origin of the previously demonstrated increases in CK-MB and CK-MM activity in some patients with cerebrovascular accidents as described in the work of Dub0 et al [2]. It is known from previous experiments that release of CK-MB from the heart m a y occur with stimulation of certain portions of the brain.
CK-MM could be released in any circumstance in which skeletal muxle d-occurs, or even with seizures which might themselves be associated with cerebrovascular accidents. Thus, either explanation might account for increases in these isoenzymes in patients with neurological diseases.