Abstract
The rate of entry of chick embryo fibroblasts (CEF) into the Sβperiod of the cell cycle is reduced by lowering the external supply of Mg^2+^ below 0.2 mM. This slowdown, which is measured by the rate of incorporation of ^3^H thymidine into DNA, can be largely reversed by doubling or tripling the concentration of Ca^2+^ in the medium, normally 1.7 mM. The Ca^2+^βinduced stimulation is shown not to depend on contaminating traces of Mg^2+^ in the added Ca^2+^. The increase in cell number in the Ca^2+^βstimulated cultures is delayed, possibly due to cell detachment. The effect of Ca^2+^ on thymidine incorporation can be simulated almost quantitatively by Sr^2+^. Ba^2+^ does not produce the effect, nor do any of the other cations tested. As little as 0.2 mM Mg^2+^ produces a full stimulation of thymidine incorporation in the absence of added Ca^2+^, and no substitute was found that is effective in the same concentration range. In short term experiments, i.e., 16 hours, even 5.0 mM Ca^2+^ cannot stimulate thymidine incorporation to the extent achieved with 0.2 mM Mg^2+^. Large amounts of Ca^2+^ or Sr^2+^ can accelerate the uptake of 2βdGlc in Mg^2+^βdeprived cultures, but they are much less efficient than Mg^2+^ in this regard also. It is suggested that Mg^2+^ is the direct intracellular effector in controlling the diverse reactions of the coordinate response, and that Ca^2+^ and Sr^2+^ act indirectly by making Mg^2+^ available to participate in these reactions.