The chemical changes taking place in ordinary hay infusions used for protozoan cultures have been described by various authors, among whom may be mentioned Peters' and Fine.2 The points of interest discussed by these two authors are the changes occurring in the titratable acidity and alkalinity of such infusions. . Changes in titratable acidity and alkalinity, however, as is well known, do not give a correct expression of the changes in concentration of hydrogen ions. Inasmuch as the hydrogen ion concentration of the medium with which organisms are in contact is of great physiological importance, it was planned to study this factor throughout the life of series of protozoan cultures, and to see in how far such results agreed with those obtained by titration methods. In the present study, therefore, determinations were made of both titratable acidity and alkalinity, as well as of hydrogen-ion concentration of series of cultures prepared in various ways. The titratable acidity was obtained by titrating 5 c.c. of culture with 0.01 N NAOH, using phenolphthalein as indicator. The titratable alkalinity was obtained by titrating 5 c.c. of cul ture with 0.0! N H2SO4, using bromphenol blue as indicator. Hy drogen-ion determinations were made by colorimetric methods, using phosphate and acetate mixtures as standards, and phenol sulfonephthalein, brom-cresol purple and methyl red as indicators. Series of cultures were prepared in various ways and determina tions carried out daily. In the case of hay infusion cultures with hay, the same amount of hay (approx. 25 gms.) was used and treated in the manner indicated in explanation of figures. The soil in soil cultures was obtained from the greenhouse and was