The influence of colostrum-derived antibody to louping-ill virus on the course of experimental infection was investigated in lambs. Lambs that had high titres of antibody were refractory to infection. Lambs that had low titres of antibody did not develop a viraemia but either showed an antibody reaction or were sensitized as judged by the immune response, which was typical of an anamnestic response, after rechallenge. Animals that had no antibody 34-20 days before challenge had either no or very slight viraemia, but did develop an antibody response with titres as high as those of control lambs by day 21. Lambs that had been negative for longer periods responded in a similar fashion to controls. These findings are discussed in relation to the occurrence of disease in lambs kept in louping-ill endemic areas. It is concluded that in such areas infections of lambs are likely to be of minor importance as a cause of mortality and of little epidemiological significance.