Control of transcription plays a critical role in the multi-step process that regulates gene expression. Understanding the regulation of gene expression therefore depends, in part, on the ability to accurately measure defined mRNA species in cell populations and tissues. In this study, the presence of a number of mRNAs putatively involved in the determination of multiple drug resistance (MDR) was investigated, using reverse transcriptase-polymerase chain reaction (RTPCR). This technique was evaluated at a semi-quantitative and a quantitative level, giving due consideration to the choice of primers for the RT and PCR reactions, controls, reproducibility of the technique, etc. mRNA levels were initially studied in sensitive cell lines and their corresponding MDR variants. Methods were then developed which enabled analysis of intact mRNA from paired normal and tumour biopsies and archival tissues (which were embedded in paraffin). Levels of two proteins (p-glycoprotein and Topoisomerase Ila) known to have a role in MDR were studied using Western blotting techniques to establish if the mRNA and corresponding protein levels correlated. MDR 1 expression was induced in cultured cells by short-term exposure to a chemotherapeutic drug. This induction corresponded to drug concentration and was associated with physiological signs of stress. A hammer-head ribozyme to MDR 1 was transfected into both a resistant cell line and a clonal subpopulation of its sensitive parental cell line (as a control). PCR (DNA) and RT-PCR (mRNA) experiments were conducted to verify the presence and successful transcription of the ribozyme in the cells. Toxicity assays were performed to establish if the presence of the ribozyme affected the MDR profile of the cell lines.