Estimation of the specific activity of radiolabeled corticotropin and insulin by precipitation.

to any single method for trace-element analyses. In regard to whether the method is applicable to serum or plasma with abnormal viscosity: our method was not developed for the analysis of serum. Plasma is the preferred sample, for reasons previously reported (1, 8). However, for samples diluted fivefold the aspiration rates for plasma and serum are identical. We have not studied plasma with abnormal viscosity. We are puzzled by Dr. Kelson’s wish for new data on the “effectsof ions on the standards.” Such studies have been repeatedly documented in the literature. We find relevance, however, in the effect of ions added to the plasma matrix, as might occur in disease states, a study which we previously reported (9) and referred to in our method. The interference from protein or other nondialyzable components of serum, which Dr. Kelson states may be expected with all direct dilution methods, was not evident in our studies. Lastly, we do agree that the “evaluation of methods is hindered by the absence of an accepted reference method” and urge that a standard reference plasma for zinc analysis be developed. We trust that our work, and indeed the work of Dr. Kelson and others, will stimulate concurrence for the necessity of a Selected Method for plasma zinc that will serve well the needs of clinical chemistry.