The present invention provides a fusion protein construct (gp41HA) consisting of the ectodomain of the HIV-1.sub.IIIB envelope glycoprotein gp41 fused to a fragment of the influenza virus HA2 hemagglutinin protein. Immunization in-vivo via an intraperitoneal prime followed by intranasal or intragastric boosts with gp41HA induces high concentrations of serum IgG antibodies and fecal IgA antibodies that reacted with gp41 in HIV-1.sub.IIIB viral lysate and are cross-reactive with gp41 in HIV-1.sub.MN lysate. Followup analyses by indirect immunofluorescence showed that both serum IgG and fecal IgA recognized human peripheral blood mononuclear cells infected with either syncytium-inducing (SI) or non-syncytium-inducing (NSI) North American HIV-1 field isolates, but not uninfected cells.