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The importance of cell surface expressed CD86 and CD83 in the regulation of immune responses such as those involved in tumour immunity has been well established. The release of soluble forms of these molecules provides a potential mechanism for modulating the signals delivered through their cell surface counterparts. There has however been little work carried out on the possibility that antigen presenting cells (APC) and/or tumour cells may release soluble molecules as a means of regulating anti-tumour responses. In this thesis the levels of soluble CD83 and CD86 molecules in patients with malignancies were analysed. An ELISA was developed to detect the circulating soluble form of CD83 (sCD83) in normal individuals and patients with various malignancies. Circulating sCD83 was detected in the sera of all of the normal individuals and patients analysed. Patients with non-haematological malignancies had levels of sCD83 within the normal range with only a small number of patients having elevated levels. Similarly, the majority of patients with haematological malignancies also had levels within the normal range, however, elevated levels of sCD83 were observed in patient's with non-Hodgkins lymphomas (NHL). Of these patients, highly elevated levels were detected in mantle cell lymphoma (MCL) patients and a proportion of chronic lymphocytic leukaemia (CLL) patients. However these trends were non-significant and further analysis of a large, number of patients within each malignant group is required to determine if the elevated levels of sCD83 are significantly higher and additionally whether any clinical conclusions can be drawn. This thesis also investigated the potential prognostic use of detection of elevated levels of the circulating soluble form of CD86 (sCD86) in patients with either acute myeloid leukaemia (AML) or myelodysplastic syndrome (MDS). Levels of sCD86 exceeding those of the normal range were detected in a proportion of patients with AML (25%) and MDS (28%). While no correlation was observed between levels of sCD86 and the outcome of patients with MDS, elevated levels of circulating sCD86 was observed to be associated with poor prognosis in patients under 60 years of age with AML (p=0.039). Additionally AML patients with levels of sCD86 greater than 6 ng/mL regardless of age, had significantly reduced survival (p=0.019). These results indicate that sCD86 levels may be a useful prognostic marker in at least some individuals with AML. Further studies into the circulating levels of sCD86 in various other malignancies may provide information on the prognostic ability of sCD86 and the function this molecule plays in an anti-tumour response.