In vitro culture of human monocytes results in a time-dependent differentiation into macrophages. Monocyte/macrophages were infected with HIV-1Ba-L at different times after isolation and subsequent culture. When 7-day macrophages were infected in the presence of antibodies to interferon-beta (IFN-beta), a significant increase in HIV-1 p24 release was observed. This effect was not detected in 1-day monocytes. Treatment of 7-day cultured macrophages with HIV-1 rgp120 resulted in resistance to vesicular stomatitis virus infection. This rgp120-induced antiviral state was neutralized in the presence of antibodies to IFN-beta. The overall results indicate that the infection of monocyte/macrophages with HIV-1 results in the induction of IFN-beta, which, in turn, inhibits HIV-1 expression in macrophages. The finding that HIV-1 itself (possibly through its gp120) can induce a potent antiviral factor (IFN-beta) in macrophages underlines the complex physiological function of these cells in maintaining normal homeostasis in vivo in response to virus infection.