The efficiency of DNA transfection into mammalian cell cultures has been monitored using a variety of reporter assays. However, the common procedures are expensive, time-consuming and usually cannot identify the transfected cell population directly. In the present communication we describe a simple, inexpensive and efficient method to directly identify DNA transfection in mammalian cells using tartrate-resistant acid phosphatase (TRAP) gene expression. The method involves the transfection of a plasmid (pCT3), which contains TRAP cDNA driven by a CMV promoter, into mammalian cells. The cells can then be stained for TRAP activity, and the transfection efficiency can be determined by simply counting the positively transfected cells in a defined area with a microscope. This method permits screening of mammalian cells for transfection efficiency in multi-well plates. After waiting 30-40 minutes to allow the TRAP assay to saturate, wells can be scored in 1-2 minutes with little difficulty in detecting the transfected cells.