The response of goats infected with caprine arthritis encephalitis virus was evaluated by estimating the binding antibodies in serum. The immunoreactive proteins of the CAEV, strain 63, were determined by immunobloting the lysate against the serum of the infected goat. The major proteins identified were of the molecular weights of 135, 120, 56, 44, 38, 28, 20 and 13 kDa. Proteins with less prominence were of the molecular weights of 96, 32 and 30 kDa. The responses to the viral proteins at several stages of the infection were examined. Antibodies to viral proteins of 135, 121 and 90 kDa were the first to appear in the goat kids that were apparently infected through breast-feeding by infected dam. The time of appearance of these antibodies in the serum varied widely between the goats from one to six months, and preceded the appearence of antibodies to other proteins by two to four weeks. Salient variations revealed in the immunoblot patterns reflect the differences in the immunological responses by the goats to the viral proteins. Littermate goat kids infected through the same dam showed slightly different responses. In the same manner, dissimilarities were observed in the patterns produced by the serums of the damand the corresponding offsprings, when run against the same virus lysate. This study indicates that some differences occur in the immunoblot pattern when this assay is used for diagnosis, although such discrepancies should not affect the interpretation of the diagnostic assay. The host differences in the response to the viral proteins also suggest the potential problems that can be encountered developing an effective vaccine for CAEV, if the binding antibody production reflects that of the neutralizing antibodies in the goats. For effective vaccine coverage, there should be little variability in the immunogenicity of candidate vaccine antigens among the hosts.