A variety of methods have been described for introducing DNA into cultured mammalian cells. Among these, calcium phosphate (CaPO4)-mediated transfection has gained wide acceptance due to the procedure's low cost, simplicity and suitability for a wide range of cell types. However, this method is limited by relatively low transfection efficiencies and poor reproducibility. To improve the utility of CaPO4 transfections, we have identified a transfection enhancer (CalPhos Maximizer) that dramatically increases the efficiency of CaPO4 transfections with mammalian cells. The CalPhos Maximizer is a cellular structural protein that improved CaPO4 transfections with all mammalian cell lines tested and yielded greater than 70% transfection efficiency in BHK-21 cells. In the presence of the CalPhos Maximizer, transfection times were decreased to as little as 2 h with no loss in performance. Because of the high transfection efficiency provided by the CalPhos Maximizer transfection system, gene expression can be assayed following overnight incubations, rather than the typical 48-72-h intervals required by conventional transfection methods. Moreover, the Maximizer reagent is compatible with, and improves the performance of, commercially available as well as home-made CaPO4 transfection systems.