The present paper deals with a colorimetric procedure for routine determinations of plasma cholinesterase activity. The modified method of Ellmanet al. (1) is described in detail and a statistical treatment of results obtained with plasma samples of almost 100 persons is presented to justify its routine application. The data were subjected to various statistical procedures and were found to compare favorably with those described by other authors. Finally the advantages of the method are discussed in terras of simplicity and accuracy.