Several observations indicate that the different effects of serum and serum components on normal versus transformed cell growth are related to divalent cation metabolism. The growth of normal cells in culture can be modified by both the availability of serum and divalent cations. There is evidence that divalent cations and serum act synergistically in the control of cell growth and that this control is lost upon transformation to the malignant state. Serum or calcium deprivation inhibits proliferation of a number of normal cells in contrast to the absence of such an effect on transformed counterparts. However, elevation of medium Ca levels in serum deprived normal cells can antagonize the growth inhibitory effect of low serum. The loss of density dependent growth inhibition in 3T3 cells is characterized by a simultaneous reduction in the extracellular Ca and serum requirement. Magnesium and serum act synergistically in inducing chicken fibroblasts to traverse G1 phase and initiate DNA synthesis. Serum platelet factors are required to commit several normal cell types to DNA synthesis but not transformed cells. Ca can substitute in part for these factors. We propose to establish the extracellular Ca, Mg and serum requirements for growth of human diploid fibroblasts as compared to an SV40 transformed counterpart with specific reference to their ability to traverse G1 phase of the cell cycle, the phase in which growth control is exerted in the normal cell. We have achieved synchronization of both the normal and transformed W138 cell using centrifugal elutriation. This will allow direct comparisons to be made between these cells using an identical synchronization procedure. We propose to evaluate what effects the manipulation of serum, Ca and Mg have on divalent cation transport and content during G1 phase progression in the normal cell and how these parameters are related to loss of growth control in the transformed cell. We propose to determine any specific changes in divalent cation management associated with the ability of platelet factor to induce competence for DNA synthesis in normal cells and how such changes might relate to the capability of transformed cells to remain continually competent in the absence of platelet factor.