The history of the colloidal gold reaction is one of great interest. In 1901 Zsigmondy used the precipitation of colloidal gold by proteins as a means of determining quantitatively their amount in given solutions. He discovered that solutions of protein give protection to colloidal solutions of gold up to a certain point, and he determined the so-called goldzahl for various protein substances, by which is meant the number of milligrammes of protein employed, just sufficient to prevent the precipitation of 10 c.c. of colloidal gold of a percentage of .0053 in the presence of 1 c.c. of 10 per cent. sodium chloride. By this method it can be determined whether a given protein is absolutely pure, or, granted that this is the case, how much of the protein is present in a given solution.