The reliability of the competitive polymerase chain reaction (competitive PCR) for the detection and quantitation of gene expression in small tumor samples was evaluated. DNA polymerase-beta gene expression was detected in human ovarian cancer cell lines displaying a different degree of cisplatin resistance. The level of DNA polymerase-beta cDNA in the resistant cell line was threefold that of the parental sensitive line. Our results indicate that competitive PCR is a reproducible and sensitive method to detect differences in gene expression in small samples and open the possibility of using this approach to detect DNA polymerase beta cDNA in small samples from clinical tumors.