Biomarkers have great potential for cancer detection and monitoring, with much of this type of analysis being carried out using tumour biopsies. This approach requires invasive procedures to obtain suitable specimens and only allows analysis of gene expression at one particular time point in the history of a cancer and from one location in the body. Biomarkers detectable in readily accessible body fluids, such as scrum, saliva or urine would allow on-going/ sequential monitoring of the course of disease (progression, response to therapy, etc.). A small number of studies have indicated the possibility of amplifying extracellular mRNA from the serum and/or plasma of cancer patients supporting the potential of this route of analysis. Limitations of these studies include the small numbers of serum/plasma specimens analysed; limited numbers of gene transcripts analysed; and discrepancies in protocols used, leading to a possibility that cells circulating in the bloodstream may be included in the RNA isolations; and the fact that a standard technique in general, was not employed. This thesis aims to address these issues by firstly establishing the possibility of routinely extracting and amplifying extracellular mRNA from the conditioned media (CM) of cultured cells. Having established, in principle, that amplifiable RNA could be isolated, comparison and optimisation of methods of extracting RNA was carried out to identify a reliable and reproducible method. Once this was established, CM samples from different cancer cell lines were investigated for expression of known tumour- related mRNAs by RTPCR and microarray technology was employed to investigate the global expression of mRNAs with both known and unknown functions. A slightly modified version of this method was also applied to mRNA extracted from serum of breast cancer patients and normal volunteers, enabling approx. 55,000 transcripts/ variants represented on the whole human genome chips to be analysed to identify mRNAs suitable for further analysis as potential future biomarkers.