We have developed a method to accurately quantitate IFN gamma mRNA in HUT-78 cells before and after PUVA treatment by the competitive RT/PCR technique, which could be utilized to accurately quantitate any mRNA species of interest. Total RNA was isolated from HUT-78 cells before and after PUVA treatment. A synthetic IFN gamma mRNA was made to contain a 54 bp deletion in the middle of IFN gamma cDNA gene and used as an internal standard. 0.5 microgram of target RNA was co-reverse transcribed and co-amplified with increasing concentrations of synthetic IFN gamma RNA using the same primers. The products of the synthetic RNA were separated from that of the target RNA by gel electrophoresis. This allowed determination of the amount of target IFN gamma mRNA to be quantitated by extrapolating against a standard curve. PUVA treatment of HUT-78 cells resulted in an increase in IFN gamma mRNA level from 32 to 80 pg/microgram of total RNA, suggesting that PUVA induces transcription of T-helper 1 cytokines as part of its mechanism of action.