Isolated HIV-1 Group O env polypeptides obtained from the HIV-1 isolate HAM112 are claimed, as well as (a) antigen constructs comprising fusions of one or more of each of HIV-1 Group O env polypeptides and HIV-1 Group M env polypeptide and (b) further antigen constructs containing additional Group O sequences and especially the gp41 IDR of isolate HAM112. Also claimed are polynucleotide sequences encoding the above, expression vectors comprising the same, host cells transformed thereby, and immunoassay methods and kits utilizing the antigen constructs of the invention.