Antigenic relationships between the four subgroups of the Kemerovo serogroup of tick-borne orbiviruses were examined. Kemerovo (KEM subgroup), Broadhaven (BRD) and Wexford (WEX) [Great Island (GI) sub-group], Chenuda (CNU sub-group), and Wad Medani (WM sub-group) viruses cross-reacted in immunofluorescence tests. Complement fixation tests (CFT) revealed that KEM virus was more closely related to BRD and WEX viruses than to either CNU or WM viruses. By cross-neutralization, all the viruses were shown to be distinct; only BRD and WEX showed slight cross-reaction. In Vero cells infected with either KEM, BRD, WEX, or CNU viruses, 10 major (p1 to p10) and a variable number of minor virus-induced 35S-labelled polypeptides were detected. Comparison of the polypeptides precipitated by homologous AF revealed close similarities between BRD and WEX viruses but obvious differences between these two viruses and KEM and CNU viruses. In cross-reactions, p6 of KEM and CNU viruses, and p7 of BRD and WEX viruses were immunoprecipitated. Immunofluorescence and immunoprecipitation tests appeared to identify an inter-group specific antigen that was distinct from an intra-group specific antigen detected by CFT. The results support division of KEM-related viruses into 3 new serogroups - KEM (comprising KEM and GI subgroups), CNU and WM - and indicate that a 38 to 43kD polypeptide carries the major inter-group specific antigen.