We have previously shown that human CD3- CD56+ and CD3+ CD56+ cells from some individuals mount vigorous proliferative responses to lipopolysaccharide. Such responses have been blocked by the presence of cytotoxic T-lymphocyte antigen-4 immunoglobulin fusion protein in the cultures, implicating a role for B7-mediated costimulation. Here we confirm this inhibition of natural killer (NK) expansion using antibodies against B7-1 and B7-2. We were unable to specifically detect CD28 on the surface of resting or stimulated human peripheral blood NK cells, however, in either lipopolysaccharide-responsive or non-responsive individuals, using a panel of four different anti-CD28 monoclonal antibodies. T-cell depletion from peripheral blood mononuclear cell cultures resulted in a reduction in the induction of CD25 on activated CD3- CD56hi cells and in the expansion and proliferation of CD3- CD56+ NK cells. Furthermore, reconstitution experiments using peripheral blood dendritic cells and purified NK cells demonstrated that NK expansion could only be achieved in the presence of purified T cells.