Reverse transcriptase-polymerase chain reaction (RT-PCR) techniques have been widely employed as an ultra-sensitive method for detection of micrometastases in patients with various types of malignancies. Messenger RNA of a specific marker gene is a target for RT-PCR amplification to examine the presence of micrometastases in body fluids or tissues obtained from human. We developed the RT-PCR assay specific for rat beta-actin mRNA, which cannot detect human counterpart and assessed how much contamination of rat tissues can influence the result of RT-PCR assay and how to avoid the influence of the contamination in RT-PCR assay.