High-yield mammalian transient expression systems can include a cell culture media (particularly serum free, non-animal derived, and/or chemically defined media) for introducing macromolecules and compounds (e.g., nucleic acid molecules) into cells (e.g., eukaryotic cells). Cells containing such introduced materials can then be cultured in the cell culture media. In particular, the invention allows introduction of nucleic acid molecules (e.g., vectors) into cells (particularly mammalian 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.