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Yields of reovirus type 3 were depressed and cytopathic changes delayed when calf or sheep serum was present inthe culture medium. Both sera also neutralised free reovirus. Fractionation of pooled serum samples by DEAE- cellulose and Sephadex G-200 chromatography showed that in thecase of calf serum the inhibitory and neutralising activities were associated with the γG globulin fractions. Therefore it is likely that the inhibitory activity ofcalf serum is due entirely to the presence of neutralising antibody. In the case of sheep serum, however, neutralising and inhibitory activities were dissociated. Virus-neutralising activity in this case was maximal in the γG globulin fractions, whilst the inhibitory activityresided in the γM fraction. The inhibitory componentof sheep serum appears to act by blocking virus release.