A method is described to deliver mRNA to plant protoplasts using polyethylene glycol for transient expression. In order to optimize delivery and subsequent expression, we have examined a number of parameters that affect expression levels including the requirement for Mg(2+) and Ca(2+) ions, carrier nucleic acid, and the linearity of the dose-response. Delivery of mRNA using PEG results in levels of expression that are comparable to those obtained with electroporation. This method is particularly useful for those species in which electroporation does not work well. Moreover, the use of PEG to deliver mRNA is an inexpensive alternative to electroporation in that it does not require an electroporator.