Methods have been developed for the quantitative assay of cytochrome oxidase, esterase, and 11 acid hydrolases in rat-spleen homogenates. These methods seem to be applicable also to other lymphoid tissues. Preliminary studies, extended to nine of the acid hydrolases, indicate that these enzymes occur in partly latent and sedimentable form and that they can be unmasked and rendered soluble by some of the treatments that liberate the enzymes from rat-liver lysosomes. The spleen particles appear to be very sensitive to mechanical injury, a property which necessitates special precautions in homogenizing the tissue. Agglutination of spleen particles takes place to a larger extent in 0.25 M sucrose than in 0.15 M KCl.