Integrating microdensitometry was used to study changes in the intracellular activity of 4 enzymes during macrophage development. Suspension cultures of blood monocytes from 19 healthy human subjects were examined at 0, 2, 4 and 6 d. Mononuclear phagocytes were harvested by glass adherence and standard methods were used for cytochemical staining for NADH dehydrogenase, succinate dehydrogenase, acid phosphatase and alpha-naphthyl butyrate esterase. All specimens from all subjects were stained at the same time and staining intensities in individual cells were measured at appropriate wavelengths. A highly significant increase in enzyme activity with culture time was found for all 4 enzymes. These increases in mitochondrial, lysosomal and ectoenzyme activities during development indicate the increasing functional capabilities of the macrophages.