Mouse adrenal tumor cells which experience a large number of serial passages lose their ability to synthesize steroid hormones in response to stimulation by adrenocorticotropic hormone (ACTH). Based on the importance of plasma membrane hormone receptors and actin-containing microfilaments in the hormone stimulation of these endocrine cells the two components of the adrenal cell membrane seemed to be likely sites for age related changes to occur. Methods are proposed to follow the biogenesis of these macromolecules in recently cultured cells. By selectively harvesting mitotic cells growing in tissue culture a synchronized population of cells is obtained which, after cytokinesis, has one-half of its original limiting membrane. This observation will be utilized to label the two membrane-associated macromolecules so that they can be isolated and characterized. The techniques which are developed will be used to compare the two membrane components in early and late passage adrenal tumor cells. In this manner, information can be obtained regarding the effects of aging on the pituitary-adrenal axis.