The purpose of this project was to examine and optimise the production and application of bispecific antibodies. A bispecific antibody is an immunoglobulin-derived molecule which binds monovalently to two distinct antigens. It can be produced using somatic cell fusion techniques or by chemical m anipulation of the parental immunoglobulin molecules. Bispecific antibodies were produced from two different species of immunoglobulin, human immunoglobulin and mouse immunoglobulin or rabbit immunoglobulin and mouse immunoglobulin. These were characterised using high performance liquid chromatography, polyacrylamide gel electrophoresis and enzyme-linked immunosorbent assays based on both parental species differences and double antigen specificity. The bispecific antibody was produced at 70% of total potential yield. Antigen specificity was retained but binding affinity was reduced by 30-50% compared to parental molecules. A rabbit, polyclonal anti-alkaline phosphatase antibody was produced. This antibody was used, in conjunction with a murine, monoclonal anti-chronic lymphocytic leukaemia antibody (G12), to produce a bispecific antibody. The bispecific antibody was characterised, as before, following purification by double affinity chromatography. The bispecific antibody was used to develop a suspension and a plate enzyme-linked immunosorbent assay for the detection of the hum an leukaemic cell line, K562. These assays were used to detect chronic lymphocytic leukaemic cells in patient blood samples. A positive diagnosis was made in 80-90% of known chronic lymphocytic leukaemic patients. The same anti-chronic lymphocytic leukaemia X anti-alkaline phosphatase bispecific antibody was used in immunocytochemical studies of the same patients blood samples, with a positive response of 75-85%. A second bispecific antibody was produced from the rabbit, polyclonal anti-alkaline phosphatase antibody and a rabbit, polyclonal anti-7-hydroxy coumarin antibody. This was characterised, as before, following purification by double affinity chromatography. The anti-7-hydroxy coumarin X anti-alkaline phosphatase bispecific antibody was used, with limited success, to develop an enzyme-linked immunosorbent assay for the quantitation of 7-hydroxy coumarin.