To investigate the resistance against 5-fluorouracil (FU), thymidylate synthase (TS) mRNA level was measured by reverse transcription PCR (RT-PCR) technique and quantitated by a competitive-PCR method in human colon adenocarcinoma cell line, SNU-C1 and its FU resistant cell line, C1/FU. Using a set of primers covering 3'-untranslated region of mRNA, PCR products were amplified from cDNAs prepared from both SNU-C1 and C1/FU. However, only a cDNA extracted from C1/FU was amplified in a stationary growth phase. Then, the amount of mRNA was quantitated by competitive-PCR technique in both cell lines using another set of primer enable to amplify product in both stationary and logarithmic cell growth phase. The amount of mRNA in C1/FU was four times more than that of C1. The gene level in TS gene in genome DNA in these cell lines were not different incidentally. These data suggest that TS mRNA was more increased and can be more stable in C1/FU, accounting for FU resistance.