1. The smallest amount of the sodium soaps necessary for the complete hemolysis of 0.5 of a cubic centimeter of a 5 per cent. suspension of the red blood corpuscles of the sheep, ox, rabbit, dog, or of man, is about the same,—0.03 of a milligram in the case of the following acids: oleic, linoleic, dibromostearic, chloriodostearic, and two isomeric monobromostearic acids; in the case of erucic acid about twice as much of the soap was found to be necessary; in that of palmitic or of dihydroxystearic acid more than ten times as much. 2. The minimum hemolytic quantity of the sodium soaps of the highly unsaturated acids obtained from cod liver oil and from linseed oil is only very slightly less than that of sodium oleate. 3. It follows, therefore, from these results that hemolysis by unsaturated fatty acids is not more active in proportion to the degree to which these acids are unsaturated, nor is it diminished when the unsaturated carbon atoms are saturated by halogens. It is, on the other hand, greatly diminished when they are converted into the corresponding hydroxyl acids, which are hemolytic only to the same degree as the saturated acids. 4. The idea that toxic hemolysis, in disease, in poisoning by phosphorus or toluylene diamine, results from the liberation of specially hemolytic fatty acids from the fatty complexes of disintegrating cells is not well supported by evidence; none of the fatty acids, still less any of the fatty complexes from which these acids can be obtained in any of the organs examined, either in this work or in the work of others that has preceded it, show on analysis any evidence for the existence of fatty acids more toxic than the common oleic acid which is constantly being set free by hydrolysis from common fat in health.