The activity of cathepsin B was assayed in human blood monocytes during differentiation into macrophages in vitro. Freshly isolated monocytes showed negligible cathepsin B activity. On day 3 in culture the enzyme activity was still very low, but it was markedly increased on day 7, concomitant with the monocytes' morphological differentiation into tissue macrophage-like cells. A further rise in enzyme activity was seen on day 10 in culture. Acid phosphatase activity showed similar, but less marked, increases in human monocytes during 10 days' culture. Generally, higher enzyme levels were seen in monocytes isolated from buffy coat preparations than from whole blood. Both the level and the rate of appearance of cathepsin B activity were further enhanced by endocytosis of carrageenan in the monocytes. Stimulation with endotoxin from Escherichia coli caused variable enzyme responses, as certain pools of human cells expressed cathepsin B activity compared with controls, whereas others showed no change in activity. Endocytosis of carrageenan had no effect on acid phosphatase activity, whereas stimulation with endotoxin led to increased levels of this enzyme activity in all cultures. The present data suggest that a rise in cathepsin B activity may be a component in the differentiation of human monocytes into macrophages. They further indicate separate regulation of lysosomal enzyme activity in human monocytes after some types of stimulation, as previously shown for mouse macrophages.