CAMPATH-1H (where CAMPATH is a trade mark of Well-come group companies), a humanized IgG antibody used in the therapy of lymphoma, leukaemia and rheumatoid arthritis, has been expressed in Chinese hamster ovary, Y0 myeloma and NS0 myeloma cell lines. These engineered cell lines were grown under different culture conditions, and the antibody isolated and purified. N-Linked oligosaccharides, on the CH₂ heavy chain region of the antibody, were isolated and analysed by hydrazlnolysis, high-performance anion-exchange chromatography with pulsed amperometric detection, laser-desorption mass spectrometry and sequential exoglycosidase treatment Both the glycosylation pattern and the biological activity of CAMPATH-1H, as measured by antibody-dependent cell-mediated cytotoxicity, were markedly affected by the cell line used to express the antibody. It is concluded that glycosylation of the antibody may be important in the clinical outcome of therapy.