The aims of this study were to compare the in vitro responses of murine lymphocytes to HgCl2 to determine the requirement for adherent cells, and the contribution that costimulation plays in T cell proliferation. The in vitro proliferative response of murine splenocytes to HgCl2 was found to be both cell concentration- and HgCl2 concentration-dependent with the greatest response occurring with 5 x 10(6) cells/ml in the presence of 10(-5) M HgCl2. Both CD4+ and CD8+ T cells proliferated in response to HgCl2, but B cells and immature T cells (thymocytes) did not. Proliferation required the presence of splenic adherent cells and was inhibited by addition of anti-IL-1 alpha antibodies. Antibodies to the other co-stimulatory molecules CD40 ligand, CD80 (B7-1), and CD86 (B7-2), although inhibitory, were less effective. Xenobiotics such as the heavy metal mercury can elicit a spectrum of immunological responses ranging from immunosuppression to autoimmunity. The most common response, in vivo and in vitro, is lymphoproliferation, which may be a prelude to immune activation. Although a number of the requirements for mercury-induced T cell proliferation in vitro have been described, the role that adherent cells play remains to be explained. The studies described here show that interaction between co-stimulatory molecules of adherent cells and mature T cells contributes to HgCl2-induced T cell proliferation. Among these co-stimulatory molecules, IL-1 appears to play an important role. The requirement for mature T cells, adherent cells, and co-stimulatory molecules argues that HgCl2-induced T cell proliferation possesses the properties of an antigen-induced response.