The general purposes of the proposed experiments are 1) to study biochemical mechanisms involved in regression of lymphoid tissues following treatments with glucocorticoid hormones, and 2) to investigate differences which constitute the determining factors in the varied responsiveness of different cell types to steroid treatment. Two contrasting cell systems are now in use in our laboratory: the first is rat liver and thymus which react oppositely to corticoid treatment; the second is two strains of mouse lymphosarcoma P1798, one of which regresses after steroid treatment, and the other steroid-resistant strain which regresses following treatment with 5-fluorouracil. Some key biochemical reactions that may be responsible for these differences will be investigated. Among these are the role of RNases and RNase inhibitor activities in different cell fractions; effects of steroid treatment on chromatin acid proteins, on mitochondrial and microsomal activities; evidence for synthesis of early inhibitor protein in lymphoid tissues and contrast of binding of corticoid among the different cell particles in the different cell types to determine sites of reaction and to try to distinguish between relevant and fortuitous reactions.