A nested reverse transcriptase (RT) PCR assay detected mRNA of the salmonid pathogen Renibacterium salmoninarum in samples of RNA extracts of between 1 and 10 cells. Total RNA was extracted from cultured bacteria, Atlantic salmon (Salmo salar L.) kidney tissue and ovarian fluid seeded with the pathogen, and kidney tissue from both experimentally challenged and commercially raised fish. Following DNase treatment, extracted RNA was amplified by both RT PCR and PCR by using primers specific for the gene encoding the major protein antigen of R. salmoninarum. A 349-bp amplicon was detected by polyacrylamide gel electrophoresis and silver stain. Inactivation of cultured bacteria by rifampin or erythromycin produced a loss of nested RT PCR mRNA detection corresponding to a loss of bacterial cell viability determined from plate counts but no loss of DNA detection by PCR. In subclinically diseased fish, nested RT PCR identified similar levels of infected fish as determined by viable pathogen culture. Higher percentages of fish testing positive were generated by PCR, particularly in samples from fish previously subjected to antibiotic chemotherapy where 93% were PCR positive, but only 7% were nested RT PCR and culture positive. PCR can generate false-positive data from amplification of target DNA from nonviable pathogen cells. Therefore, nested RT PCR may prove useful for monitoring cultured Atlantic salmon for the presence of viable R. salmoninarum within a useful time frame, particularly samples from broodstock where antibiotic chemotherapy is used prior to spawning to reduce vertical pathogen transmission.