In order to investigate the age-dependent increase in the level of chromosomal protease in thymus but not in others the following experiments will be carried out: 1) The amount of chromosomal protease in the isolated thymocytes will be titrated as a function of age. 2) As model system for aging thymus the effect of hydrocortisone and X-ray irradiation on the chromosomal protease in the thymus of mice and rats will be studied in detail. Also the increase in nucleases will further be investigated. 3) The possible presence of gene-specific proteins in the nonhistone proteins remain bound to the DNA in 2M NaCl and the implication of the age-dependent increase in the nonhistone proteins bound to the DNA in 2M NaCl will be studied. 4) Purification of the thymus chromosomal protease by affinity chromatography will be continued.