Untransformed NIH 3T3 cells do not proliferate in media with reduced calcium, while SV40-transformed NIH 3T3 cells do. Intracellular calcium stores of untransformed cells were depleted to a higher extent than those of transformed cells under these conditions, which led to a decreased intracellular calcium transient in response to serum, compared to SV40-transformed cells. Furthermore, untransformed cells could be gradually adapted to proliferate in the low-calcium medium and, after adaptation, maintained their stores and serum response in low calcium media. Our experiments indicate that it is the ability of the cells to maintain adequate calcium stores in low calcium media that correlates with a full serum response and the ability to proliferate, rather than any differences reflected in alterations of resting calcium levels.