Matrilysin is a member of a multigene family of proteolytic enzymes called the matrix metalloproteinases (MMPs). It has previously been determined that matrilysin has a role to play in the development of colon cancers, however, its participation in other tumour types has not been fully explored. The aims of this research were (i) to develop an ELISA for matrilysin; (ii) to examine human breast tumour tissue by a variety of methods for expression of matrilysin; (iii) to investigate the expression of matrilysin in cancers of other tissues using cell lines; and (iv) to examine how the expression of matrilysin could be modulated by polypeptide growth factors and cytokines. A sensitive and specific one-step sandwich ELISA was developed. This was validated and shown to have excellent reproducibility. The limit of detection was 0.45 ng/ml and the linear range was 5 - 5 0 ng/ml. A pilot-scale study of ten breast tumour samples and three normal breast tissue samples demonstrated the presence of matrilysin mRNA and protein in both normal and tumour cells. Immunohistochemistry was found to be the most useful and informative method of detecting matrilysin protein. The intensity of expression varied among tumour specimens but did not correlate with stage of disease. Matrilysin expression was examined in a variety of human tumour cell lines using RTPCR and the matrilysin ELISA. Of the eight lines tested, only four produced the enzyme. Levels of matrilysin could be stimulated with cytokines. Interleukin-6, IGF-I and IGF-II, not previously reported as modulators of MMP activity, were among the most potent stimulators at both the mRNA and protein levels. These results were confirmed using matrilysin promoter-reporter gene constructs transfected into suitable cells.