Gene delivery is an important step in the preparation of cell arrays. A highly efficient delivery system is required to modulate the cellular functions for analysis. In this study, branched polyethylenimine (bPEI), a very potent and cost-effective polymer for transfection, was used as a primary carrier for gene delivery. The percentage of cells expressing green fluorescent protein was used to indicate the transfection efficiency. Various substrates were coated on slides for the examination of cell growth and transfection efficiency. Different enhancers, such as polyethylene glycol, dextran, and gelatin, were combined with the transfection reagent in an attempt to increase the transfection efficiency. For the slides coated with gelatin type B that could provide better cell growth, the transfection efficiency was observed to strongly depend on the amount of plasmid contained within the transfection reagent in a dosage response. In the absence of any enhancers, the efficiency reached 20%, whereas in the presence of polyethylene glycol of MW 2000 or gelatin type A, the efficiency could be further enhanced to around 70%. When bPEI was replaced with dextrangrafted bPEI, a more efficient carrier for transfection in cell culture, very minimal levels of GEP positive cells were detected, suggesting that different factors might exist in transfecting cells in culture and those in microarrays for PEI-based vectors to achieve high efficiency. This Study showed that using bPEI combined with an enhancer as a gene delivery system could efficiently modulate cellular functions for the preparation of cell arrays. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 2221-2225, 2009