Immunological cross-reactivity between six-helix bundle structures derived from HIV-1 IIIB and BaL. Serial two-fold dilutions of mAb NC-1, raised against HIV-1 IIIB gp41 six-helix bundles, were added to wells coated with the respective N36/C34 peptide complexes [20, 21] and binding of the IgG antibodies was determined as described in Methods. Normal mouse IgG was used in control experiments instead of mAb NC-1. Dilutions of the respective gp41 six-helix bundles were also tested in a sandwich ELISA (insert). The absorbance for controls in the absence of gp41 bundles was 0.