Monospecific polyclonal antisera were raised in guinea-pigs against the calf rotavirus polypeptides VP1, VP2, VP3 + 4, VP4.2, VP6, VP7.1, VP7.2 and VP10. All of the antisera gave a similar pattern of cytoplasmic immunofluorescence in rotavirus-infected cells, but spots of fluorescence of varying intensity with different sera were seen over the nucleus. Immune precipitation, using Staphylococcus aureus to collect immune complexes, showed that VP2 was precipitated by antiserum to VP2 (alpha-VP2) and VP6 by alpha-VP6, alpha-VP7.1 and alpha-VP7.2 both precipitated the same range of proteins from infected cells (VP7, VP7.1 and VP7.2) or from virions (VP7.1 and VP7.2). VP10, either from virions or infected cells, was not precipitated by alpha-VP10. The only antiserum which efficiently neutralized infectivity was alpha-VP7.2. There were low levels of neutralization with alpha-VP10 (but the results varied from experiment to experiment) and traces with alpha-VP6. alpha-VP7.1 and the other antisera did not neutralize even though alpha-VP7.1 agglutinated double-shelled particles as seen in immune electron microscopy to a greater extent than alpha-VP7.2. Both VP7.1 and VP7.2 were shown to be glycoproteins by tunicamycin treatment of infected cells. Core particles only were agglutinated by alpha-VP10. All the evidence leads us to conclude that there were major neutralizing antigenic determinants present on VP7.2, a minor component of the outer shell of the virion.