An immunoassay diagnostic kit for the detection of antibodies in a test sample comprising an antigen consisting of a self-assembled, non-infectious, non-replicating, immunogenic, retrovirus-like particle encoded by a modified HIV genome devoid of long terminal repeats containing a nucleotide sequence coding for a chimeric envelope glycoprotein, said chimeric envelope glycoprotein having a first retroviral envelope amino acid sequence and a second retroviral envelope amino acid sequence, wherein said first amino acid sequence contains the HIV-1 gp120 conserved region 2 and said second amino acid sequence contains a retroviral envelope amino acid sequence of a heterologous strain of HIV-1, HIV-2, HTLV-I, or HTLV-II inserted into said first retroviral envelope amino acid sequence by inserting a nucleotide sequence encoding said second retroviral envelope amino acid sequence into a nucleotide sequence encoding said first retroviral envelope amino acid sequence at an endogenous conserved region 2 restriction site selected from the group consisting of BglII and StuI.