Gold sols for use in the colloidal gold reaction prepd. by reduction with Na citrate have several advantages over the neutral formol gold, which, although effective, is difficult to prepare and is subject to unsatisfactory modifications. Gold sols obtained with a conc. of Na citrate of 1:10,000 have a similar sensitivity, at pH 7.4 of the milieu, to that of formol gold. This . is about the optimal sensitivity; gold sols prepd. with higher conc. of citrate, which are of an orange-red color, are unsatisfactory for purposes of the gold reaction. Citrate gold 1:10,000 is acid. The simplest way to neutralize this acidity in order to maintain the obligatory pH of the milieu, around 7.4, consists in the use of strong buffers as diluent, instead of saline. The simple detn. of the turbidity of citrate gold 1:10,000 with the photoelectric colorimeter provides an accurate and objective control of its repro-ducibility. The progress of aging may be similarly checked. By this method of control, identical gold sols can be employed in different laboratories. The combination of standardized gold sol and buffers permits the results to be recorded quantitatively with an easily prepd. gold standard, thus providing a thoroughly controlled quantitative gold reaction.