Preliminary results have shown that: 1) in a small proportion of cytochalasin b-(CB) induced binucleate WI-38 cells, one of the daughter nuclei tranvesed the G1 independently of the sister nucleus and entered S phase, and 2) the G1 nucleus is refractory to induces of nuclear DNA synthesis generated by sister nucleus residing in the same cell cytoplasm. The specific aims are to study whether: I) nuclei of non-dividing cells (G1) arising at different population doubling levels upon fusion respond to inducers of nuclear DNA synthesis generated by normal cells in S phase and initiate DNA synthesis; II) inducers of DNA sy nthesis are the limiting factors in the initiation of DNA synthesis in non-dividing cells; III) the G1 nucleus in CB induced binucleate cells containing g1/S components slows down DNA synthesis in the S phase component; IV) non-dividing cells eventually develop cellular mechanisms of inhibition of DNA synthesis upon fusion to normal cells in S phase; V) normal cells arrested in G1 phase of the cell cycle upon fusion inhibit DNA synthesis in young fibroblast nuclei. The methodologies include: i) the selection of non-dividing cells by exposing populations of WI-38 cells to bromodeoxyuridine to kill all cells traversing the cell cycle; ii) production of CB-induced binucleate cells from synchronized mitotic WI-38 cells; iii) fusion of chick erythrocy tes to CB-induced binucleates containing a G/S nucleus; iv) fusion of serum starved cells and senescent cells separately to normal cells and HeLa cells synchronized for S phase and v) monitor the coordination of DNA synthesis by 3H TdR incorporation and autoradiography. The long term objectives are to show that: a) in the cell cycle of WI-38 cells, there is a failure of positive cellular control mechanisms in the initiation of DNA synthesis; b) these cells in which this failure occur become non-dividing cells and are arrested in G1 phase and c) these cells eventually develop negative cellular control mechanisms of inhibition of DNA synthesis in normal cells in S phase upon fusion. The health relatedness of this project is in studies of cellular aging.