A field outbreak of Border disease (BD) on a farm in Scotland is described. Forty to fifty (11 to 14 per cent) cases of clinical BD occurred in the progeny of 350 gimmers whilst no cases were born to the 550 older ewes on the farm. Although the number of affected lambs is unusually high for Scotland, on the basis of the serology of the flock the number of infected ewes (gimmers) was considerably higher (70 per cent). Evidence is presented that once the ewe comes in contact with the BD virus it develops neutralizing antibodies against both the BD cytopathic virus isolated from the IIB pool and against the NADL strain of bovine virus diarrhoea (BVD) and both antibodies persist at least 8 months. Mean neutralizing titres in this outbreak against BVD were 8-fold higher than the titres against the cytopathic BD virus. Sero-negative ewes challenged with the IIB pool produced a 100-fold higher titre against the BD cytopathic virus than against the NADL strain of BVD virus. It is suggested therefore that the BD virus present in the IIB pool is antigenically different from that which caused this natural outbreak, and thus more than one antigenic strain of BD virus exists. Eighty per cent of sero-positive ewes challenged in the subsequent pregnancy with the BD pool were susceptible and this high susceptibility may be explained as due either to very potent inoculum, the route of challenge or alternatively the antigenic difference between the strain BD in the challenge and that causing the outbreak.