Methods using polymerase chain reaction (PCR) and gene probes to detect viable Legionella pneumophila were investigated with cells exposed to biocide or elevated temperature. Exposure to hypochlorite caused viable nonculturable cells to form. Culturable and viable nonculturable cells showed positive PCR amplification, whereas nonviable cells did not. Viable cells were also specifically detected with mip mRNA as the target, reverse transcription (to form cDNA), and PCR amplification. After exposure to elevated temperature, only viable culturable cells were detected, which corresponded with positive PCR amplification.