A modification to the existing calcium phosphate transfection method is described which yields a much higher percentage of stably transformed cells than previously obtainable. The new technique gives an expression rate of the transfected DNA of 10-50% of the cell population. It was found that the transforming marker DNA greatly influenced the transformation frequencies obtained. The method used plasmids containing a neo coding region with a suitable promoter. The quality of the plasmid DNA used for transfection also affected the efficiency of transfection, and so the standard lysozyme-triton method of DNA extraction was modified slightly. The main change to the standard transfection protocol was the way in which the calcium phosphate-DNA precipitate was formed. A buffer with a lower than usual pH was used, and this slowed down the precipitation process. The calcium phosphate-DNA complex formed gradually during an overnight incubation with the cells, and precipitated slowly onto the cells. The form and amount of DNA used and the CO₂ levels in the incubator were also found to be critical for efficient stable transformations.