A cell that is useful in production of quantities of a monoclonal antibody; a fusion of a myeloma cell and a spleen lymphocyte that, like the myeloma cell, can divide in culture and, like the spleen cell, can produce antibodies. The use of HAT (hypoxanthine/ aminopterin/thymidine) medium allows selection of hybrids of myeloma cell mutants that lack hypoxanthine-guanine phosphoribosyltransferase and consequently cannot utilise hypoxanthine, with spleen cells, which do not proliferate in vitro.