The present invention is directed generally to cell culture media (particularly serum free, non animal derived, and/or chemically defined media) which are useful for introducing macromolecules and compounds (e.g., nucleic acid molecules) into cells (e.g., eukaryotic cells). According to the invention, such introduction can take place in the presence of said medium. Cells containing such introduced materials can then be cultured in the medium and the effect of the introduced materials on the cells can be measured or determined. In particular, the invention allows introduction of nucleic acid molecules (e.g., vectors) into cells (particularly eukaryotic cells) and expression of proteins encoded by the nucleic acid molecules in the cells. The invention obviates the need to change the cell culture medium each time a different procedure is performed with the cells (e.g., culturing cells vs. transfecting cells). The invention also relates to compositions and kits useful for culturing and transforming/transfecting cells.