Mononuclear phagocyte system (MPS) is a heterogenous group of cell populations present in most tissues even in the absence of inflammation. Monocytes and Mφ are the major differentiated cells of MPS, have a prominent role in defence against many infectious agents and tumour cells, and the regulation and induction of immune responses. In addition they secrete a large number of substances which have a role in various physiological and pathological processes. It is necessary to know the normal distribution and localisation of cells with a particular phenotype in order to understand their involvement in various disease processes. The advent of hybridoma technology has facilitated dissection of the phenotypic and functional heterogeneity of various cells by producing monoclonal antibodies (mAb) specific to cell surface molecules. The sheep is emerging as an important experimental animal model for the study of the pathogenesis of infectious diseases and little is known about the cell surface determinants of MPS in this species. In this thesis I have attempted to characterise MPS by producing novel anti-Mφ mAb and further characterising anti-cattle mAb which cross react with sheep Mφ. Three mAb (VPM65, 66 and 67) immunoprecipitated cells surface glycoprotein of same M_r = 55,000, having approximately 3,000 Da N-linked glycosylation. The antigen is primarily expressed in blood monocytes in addition to its moderate expression in AM and granulocytes but weak expression in afferent dendritic cells (ADC). They also labelled resident Mφ in many different tissues but were not reactive with lymphocytes. The molecule is anchored to the cell surface via glycosyl-phosphatidyl inositol linkage. These mAb recognise the same or overlapping epitopes of the antigen. VPM65/66/67 recognise an homologue of sheep CD14 as determined by antigen preclearing studies using human anti-CD14 mAb (TuK4). These mAb also reacted with monocytes and Mφ in cattle.