The present invention relates to the diagnosis of HIV infections. It especially teaches the production of a soluble retroviral surface glycoprotein- (or transmembrane glycoprotein)-chaperone complex and the advantageous use of a chaperone-antigen complex especially in the detection of antibodies to HIV in immunoassays, preferably according to the double antigen bridge concept, or as an immunogen. The invention also discloses soluble complexes comprising a variant of HIV-1 gp41 or a variant of HIV-2 gp36, respectively, and a chaperone selected from the peptidyl-prolyl-isomerase class of chaperones. Variants comprising specific amino-acid substitutions in the N-helical domain of HIV-1 gp41 or of HIV-2 gp36, respectively, are also described.