Subcellular fractions were isolated by differential centrifugation from pure suspensions of human blood lymphocytes incubated with and without phytohemagglutinin (PHA). Between 30 and 120 min after addition of PHA to intact cells, redistribution of acid hydrolases (beta glucuronidase, acid phosphatase), from a 20,000 g x 20 min granular fraction into the corresponding supernatant, was observed. No increase in total acid hydrolase activity was found at these times. The mitochondrial marker enzyme, malate dehydrogenase, did not undergo redistribution. Granules derived from PHA-treated cells became more fragile upon subsequent incubation with membrane-disruptive agents in vitro (streptolysin S, filipin). These changes were associated with an increase in the over-all permeability of the stimulated cell to substances in the surrounding medium, such as neutral red. Augmentation of dye entry into lymphocytes required intact metabolism as judged by response to temperature and inhibitors (cyanide, antimycin A, 2,4-dinitrophenol). PHA, however, did not release enzyme activity from hydrolase-rich granules in vitro or render them more susceptible to subsequent challenge with membrane-disruptive agents. These studies suggest that PHA induces early changes in the surface of lymphocytes. The consequent redistribution of acid hydrolases may play a role in remodeling processes of the stimulated cells.