Recent studies with endothelial cells and epithelial cells show that denial of anchorage causes apoptosis whereas fibroblasts were unaffected. In our laboratory, however, we find that a combination of growth factor withdrawal and denial of anchorage induces apoptosis in human diploid fibroblasts. We propose that the two signals are required for apoptosis and that one signal alone leads to cellular senescence rather than apoptosis. Using recently developed FCM and biochemical techniques we will define the signal transduction pathways that lead to cellular senescence and apoptosis in human diploid fibroblasts. The final goal of this project is to investigate how denial of anchorage is related to the regulation of the cell cycle.