The acid phosphatase and beta glucuronidase activities of four subcellular fractions (nuclear, mitochondrial-lysosomal, microsomal, supernatant) of WI-38 cells were compared during in vitro aging. All of the fractions showed an age-associated increase in activity. The increase in the lysosomal fraction was sufficient to account for the increase in the whole homogenate. The supernatant fraction showed a consistent and pronounced increase suggesting a decrease in latency. Hydrocortisone stabilized the lysosomes to some extent. However the presence of hydrocortisone (5 mug/ml) in the growth medium consistently extended the life-span of the culture 20-30%. The magnitude of the extension seemed to be directly proportional to the amount of time the cultures were exposed to the added hormone.