We have developed reverse transcription-polymerase chain reaction (RT- PCR) methods for detecting prostate-specific antigen (PSA) mRNA. Using these methods, and a highly sensitive immunofluorometric assay for measuring PSA protein, we have assessed the concentrations of PSA mRNA and PSA protein in 30 primary breast tumors and a few other control tissues. We found good agreement between presence of PSA protein and PSA mRNA in breast tumors. We thus propose that, in women, detection of PSA protein or PSA mRNA in tissues and tumors offers equivalent information. Because PSA protein is present in male blood and thus could contaminate extracts from tumors and tissues from men, we propose that the RT-PCR methods we describe be used to assess nonprostatic expression of the PSA gene in men.