OBJECTIVE: To assess whether the hormone dependent regression of MTW9 mammary carcinoma can be explained by an increased lability of cytosol proteins to lytic enzymes. The work of last year demonstrated that one of the earliest events in tumor regression was in increased lability of cytosol proteins to lytic enzymes. The objective of this year's work was to establish whether a change in the protein pattern of the cytosol appeared after hormonal removal. Arrest of synthesis of 3 proteins has been observed within hours from hormonal deprivation despite continuation of total protein synthesis by the tumor at levels similar to the growing phase. BIBLIOGRAPHIC REFERENCE: Rouleau, M. and Gullino, P.M.: Increased Susceptibility of Cytosol Proteins to Proteolytic Digestion during Regression of a Hormone-dependent Mammary Tumor. Cancer Res. 37: 670-677, 1977.