Extracellular magnesium (Mg) depletion inhibits the growth of the HC11 normal mammary epithelial cells. In this study we found that an acute increase in extracellular Mg generally exerts a positive effect on the growth of these cells. We also isolated and characterized two derivatives adapted to grow and proliferate at nonphysiological concentration of Mg. The growth properties of the HC-LMg cells at 25 muM Mg were comparable to those of the parental HC11 cells in the regular medium (0.5 mM Mg) despite an increased expression of the CDK inhibitor p27(Kip1). They also showed a reduced dependence from serum to grow. The HC-HMg cells have been adapted to grow and proliferate at an increased (45 mM) Mg concentration. Cell total Mg content was 19.6, 9.7, and 20.1 nmol/mg protein in the HC11, HC-LMg, and HC-HMg cells, respectively. Thus, we have isolated derivatives of normal epithelial cells which are able to maintain Mg content in a physiological range in the face of different extracellular concentration gradients and will be a valuable tool for further studies on the regulation of Mg homeostasis in eukaryotic cells.