The present paper is the first of a series of reports on the relative reaction of living tissues as determined by vital staining with indicators. It is possible to bring about a localized and a general coloration of living rats and mice with litmus. The animals remain in good health and the coloration of some of the tissues persists for months. Much of the dye is stored in cell granules, especially in those of the reticulo-endothelial elements, but a diffuse staining of certain tissues occurs, notably of bone, epidermis, cartilage, and connective tissue everywhere. In the intensity and localization of the bony coloration litmus has resemblances to madder. Diffuse staining with it renders blue most, if not all, of the tissues affected, while a granular staining causes others to become notably pink, owing to the fact that the indicator, though introduced into the organism in the blue form and circulating as such in the body fluids, is ordinarily red when stored in cells. The polymorphonuclear elements and macrophages of a peritoneal exudate, may become so laden with material colored red by litmus that the blue color of the fluid constituent is masked and the exudate appears a deep, turbid red. The phenomenon is but one manifestation of a notable acidity within cell granules throughout the organism. Like many another in the stained animals it would appear to be of physiological import. Some of the questions suggested by the work will be dealt with in the paper immediately following.