Purified human peripheral blood lymphocytes have been shown to develop acid hydrolase-rich granules between 24 and 48 hours after stimulation by phytohemagglutinin, prior to mitosis. This increase has been measured biochemically as a net increase in total activity of the lysosomal enzymes: acid β-glycerophosphatase, acid phenolpthalein phosphatase, and aryl sulfatase. The subcellular localization of acid hydrolytic enzyme activities has been investigated, and they have been shown to be concentrated in a large granule fraction of sucrose homogenates and to behave as if they were membrane-bounded, in that their activity could be released by lysolecithin. It has also been demonstrated by histochemical technics that stimulation of lymphocytes by antigen (PPD) and by streptolysin S, as well as by phytohemagglutinin, produced an increase in acid phosphatase activity. Chloroquine, an inhibitor of the response to phytohemagglutinin, has also been shown to inhibit the development of acid phosphatase activity. These results are interpreted to suggest that both specific and nonspecific stimulants of lymphocytes induce lysosome-like structures in premitotic cells.