- A method is developed for PCR assay of mRNA using 28S rRNA as internal standard and its applicability is reported for 4 types of mRNA. It consists of reverse transcription of total RNA using primers selective for 28S rRNA and the message of interest. Routinely the 28S reaction mix is diluted 20,000-fold and the dilution error is minimized by adding 10 microCi of 3H2O before the dilution and determining the level of 3H before and after the dilution. Aliquots from the diluted 28S mix and the undiluted reverse transcripts are co-amplified by PCR in the presence of 32P-dATP. The samples are analyzed by electrophoresis, bands detected by autoradiography and in each lane the ratio is determined between cpm in the band of interest and cpm in the 28S band. The ratio is a measure of the level of mRNA in the sample and can be used to monitor changes in mRNA during treatments, development and pathogenesis.