We determined human Fas messenger RNA (mRNA) levels in HeLa cells using a 'mutagenic' reverse transcription-polymerase chain reaction, which quantitates mRNA levels using the corresponding genomic DNA as an internal control. The expression level of Fas mRNA was very low in serum-deprived quiescent HeLa cells. In conjunction with the start of cell-cycle progression upon the addition of serum to culture medium, the Fas mRNA level gradually increased, reached its peak at 36 h and returned to the basal level after 48 h. HeLa cells at 36 h exhibiting a high level of Fas mRNA expression were more susceptible to the anti-Fas antibody apoptotic signal. Thus, the regulation of Fas expression is associated with cell-cycle progression, and this method for Fas mRNA detection may be useful, particularly for the analysis of small amounts of samples.