While a satisfactory complement- fixation test has been developed for louping -ill disease in sheep, complement - fixing antibody is only of limited value in determining susceptibility, or immunity resulting from infection. Susceptible sheep are always negative to the test but those which recover from the disease are not always positive due to the delayed appearance, borderline titres, or rapid decline and disappearance of antibody. In contrast, serum neutralising antibody is a reliable diagnostic criterion. This antibody is stable in the recovered animal. Louping -ill mouse brain virus is accurately titrated by its cytopathogenic effect in pig kidney mor.olayer tissue culture, and sheep antiserum by its inhibitory action on virus cytopathogenicity. Such titrations are as accurate as those in mice.