With many viruses, vaccines containing the appropriate envelope antigens have provided strong and long lasting immunity. Not so with HIV-1 envelope, despite two decades of experience with various envelope and core constituent vaccines, protection provided has been weak or absent. Our laboratory has been systematically investigating the characteristics of HIV-1 envelope gp140, the principle HIV-1 envelope protein heterodimer responsible for HIV infectivity. We have identified two properties of HIV-1 envelope gp140 that may be important factors in reducing immunogenicity. HIV envelope protein gp140 rejects complement binding. Such binding can be of vital importance, since an extensive literature suggests that complement binding markedly increases immunogenicity, and, more importantly, complement binding influences the type of immune response. For many antigens, C3 binding is required for normal transport of antigens into follicles to initiate a normal germinal center response, and in the absence of appropriate complement binding, the antibody response is reduced, short lived with short-lived memory cell formation, and for an unknown reason, the antibody response shows increased affinity maturation of antibody. These features are characteristic of the HIV-1 antibody response. Just as important is the finding that envelope gp140 is highly unstable on injection, is rapidly removed from the circulation, and is degraded into peptides. This short-lived antigen may be available on initial exposure to the immune system for too short a period of time, particularly in the absence of complement binding, to be an adequate immunogen.