The distribution of glycoprotein B (gB) among different human herpesvirus 6 (HHV-6) strains was analysed with a panel of three monoclonal antibodies (MAbs) derived from mice immunized with U1102-infected lymphocytes. MAb 2D9 reacted specifically by immunofluorescence and immunoprecipitation with the U1102 and GS isolates, and failed to react with Z29 and the variant B strains Hashimoto and SF. In addition, Z29, Hashimoto and SF gB had a lower M(r) than U1102 and GS gB. MAb 2D9 also failed to react with the exanthem subitum isolate CV, included in this study as an as yet poorly characterized isolate. Consistent with this result, CV failed to react with the variant A-specific MAb to gp82-105 and behaved as a variant B virus even with respect to the diagnostic HindIII endonuclease restriction cleavage site located in a fragment hybridizing to the pZVH14 probe. By contrast with MAb 2D9, MAbs 2B9 and 2D10 reacted with all of the isolates tested, strengthening the argument tha they have common epitopes. Based on the antigenic and M(r) specificities of gB, the HHV-6 isolates tested were arranged into two non-overlapping clusters, which closely parallel the variant A and B strain groups, defined previously by several criteria, including restriction endonuclease polymorphism, antigenic variations, growth in in vitro cultures and sequence analyses.