The detection of the multridrug resistance-associated proteins is becoming increasingly important in assessing tumor sensitivity to treatment. In this work we describe a new, rapid, sensitive, and robust method for the detection of MRP1 expression based on direct RT-in situ-PCR technology and fluorochrome-modified (dCTP(Cy3)) nucleotides. MRP1 expression was found in both placenta (BeWo) and liver (Hep Ga) derived tumor cell line as well as in small cell lung carcinoma. In liver-derived cells, MRP1 expression was detected by RT-in situ-PCR but not by in situ hybridization, suggesting a higher sensitivity of in situ amplification for the low level of expression in Hep G2 cells. RT solution PCR confirmed the presence of MRP1 in BeWo and Hep G2 cells, although the level of the gene expression was lower in liver cells. This method represents a viable alternative to conventional immunohistochemistry, and may be useful in the evaluation of MRP1 expression in different tissue or cell lines.