Histochemical staining for alpha-naphthyl (non-specific) esterase has been employed to define subpopulations of human peripheral blood lymphocytes and monocytes. To define optimal conditions for staining, various fixatives, incubation times and cell preparations were compared. The great majority of human blood lymphocytes were found to have discrete granules of reaction product. More than 80% of T lymphocytes separated by E rosetting are esterase-positive whereas non-T lymphocytes are esterase-negative. Lymphocytes transformed by polyclonal mitogens lose their esterase-staining granules, which suggests that immature T cells are esterase-negative. Most blood monocytes show a diffuse cytoplasmic esterase reaction product and are phagocytic. However, about 20% of diffusely stained cells are not phagocytic. When leucocytes are cultured for 24 to 48 hr, the total number of esterase-positive cells increases and the great majority are phagocytic. This is interpreted as maturation of precursors into mature esterase-positive phagocytic monocytes. When cultured for longer periods, some lose phagocytic capacity and acquire the characteristics of secretory cells. Esterase-staining of lymph node sections allowed the identification of T- and B-dependent areas as well as macrophages related to sinuses. The esterase-staining technique could play a useful role in clinical and experimental immunology.