Dendritic cells (DC) are potent at presenting viral antigens in the initiation of both primary and secondary responses. DC in the lymph nodes draining the site of infection with HSV express surface antigen and can stimulate proliferation of sensitised lymphocytes. In secondary stimulation, nonresponsiveness in cytotoxic T cell assays of cells from mice primed to Moloney virus was also overcome by stimulation in vitro with virus-pulsed DC. Marked primary proliferative and cytotoxic T cell responses were previously found only to alloantigens and to haptens, both presented on the surface of DC However, DC exposed in vitro or in vivo to influenza virus stimulated primary proliferative and cytotoxic T cell responses in normal syngeneic lymphocytes in 20-microliters hanging drop cultures (Macatonia, Taylor, Knight and Askonas, in press). This provides a method for analysing primary responses to viruses in vitro without the necessity of using pre-sensitised donors. Although DC may present HIV antigens to lymphocytes the DC are also susceptible to infection with HIV. This occurs in vivo as evidenced by the infection of Langerhans cells in AIDS patients. This infection of DC may not only compromise their function in antigen presentation but also act as a reservoir of virus which is handed on to T cells during the close clustering of the presenting cells with the T cells during the initiation of the immune response.(ABSTRACT TRUNCATED AT 250 WORDS)