To determine the minimum amt. of invert soap required for the precipitation, an aq. soln. of the protein, containing 120-180 mg. % N, is prepared. The pH of the soln, is adjusted to the expected precipitation end point, or, in doubtful cases, to about 9. Two cc. of this soln. are placed in each of 10 centrifuge tubes, and to the first tube are added 0.5 cc. H2O and 1.5 cc. invert soap soln. (1.5% Zephirol soln.), to the second tube, 0.6 cc. H2O and 1.4 cc. Zephirol soln., etc. After the tubes have stood 18 hrs., the minimum amt. of invert soap needed for precipitation is detd. To plot the precipitation curve by method A, 50 cc. of the protein soln. containing about 150 mg. % N, are treated with the required amt. of Zephirol and enough HCl so that no precipitation occurs. 20% NaOH is then added drop by drop until precipitation is complete. Samples of the mixture are removed at intervals, centrifuged after 18 hrs., the pH of the supernatant soln. detd. by the glass electrode, and the N conc. by refractometer and by Kjeldahl. In method B, 2 cc. of the protein soln. are placed in each of 30 centrifuge tubes, 1 cc. of various concs. of acid and alkali and 1 cc. of the required invert soap soln. added. After 18 hrs., the pH and N conc. in each soln. are detd.as in method A. From a consideration of these invert soap-precipitation curves, the number, quantity and purity of proteins in a mixture may be detd. The form and pH position of the curve gives information concerning the magnitude of the dissociation constant of the positive charge carrier of that variety of protein.