The molecular sizes of the capsid polypeptides of different serotypes of bluetongue virus (BTV) have been compared to those of serotype 10A, after electrophoretic fractionation on SDS-containing polyacrylamide gels. Computer-programmed dis criminant analyses of the polypeptides’ migration distances indicated significant differences between serotypes. The results obtained are in agreement with those of molecular hybridization studies of the viral genome segments, suggesting that the main determinants of its serological specificity reside in the two polypeptides forming the outer coat of the viru