Recent studies suggest that immunity to the HIV-1 rgp120 protein may prevent primary infection. MN rgp120 vaccine and IIIB rgp120 vaccine are both prepared by recombinant DNA technology. Because the two vaccines are derived from distinct HIV-1 strains, they may elicit some immunologic responses that differ. Unlike IIIB rgp120 vaccine, the MN rgp120 vaccine has not yet been evaluated in humans, although it is expected that the MN type will result in similar safety and immunogenicity as the IIIB type.