The location of structures within an organism and the chemical nature of these structures can be determined by cytochemical techniques. Further, such techniques may be used to demonstrate the presence of an enzyme or an enzyme system within the cell. This information can assist in the elucidation of metabolic pathways available to an organism. Despite the apparently large potential value of cytochemical information, few investigations of the cytochemistry of trypanosomes have been reported, and there are no reports available of studies upon cultural forms. Nigrelli (1929) utilized vital stains and osmic acid impregnation in studies on cytoplasmic inclusions in Trypanosoma diemyctli, a parasite of the American newt Diemyctylus viridescens. He noted, particularly, the rapid appearance of well-defined “primary” granules in the region of the kinetoplast and the more gradual appearance of “secondary,” or included, granules scattered throughout the cytoplasm with an increase in time, temperature, and dye concentration. Barrow (1954) in further studies on T. diemyctil made use of the enzymes ribonuclease (RNAase) and deoxyribonuclease (DNAase) to demonstrate the distribution of nucleic acids in the cytoplasmic inclusions and in the nucleus. Sen (1930) reported a technique for locating the enzyme urease within a tissue. His method depends on the action of urease contained within the cell upon urea, which is supplied externally in the test, to liberate carbon dioxide. The carbon dioxide is precipitated as cobalt carbonate. The carbonate is then converted to the sulfide, which may be detected visually as small black dots within the cell. Although Sen described the use of his procedure on both plant and animal tissues, its use has not previously been reported in work upon trypanosomes.