Tumour necrosis factor (TNF alpha) is a major inflammatory cytokine with potentiating effects on specific immune responses, including graft-versus-host disease. This study examined the contribution of TNF alpha to dendritic cell (DC)-mediated primary allogeneic T lymphocyte responses. Purified blood DC were shown to produce minimal amounts of TNF alpha mRNA but no significant TNF biological activity or secreted TNF alpha as measured by ELISA. Amplification of DC mRNA by PCR using oligonucleotide primers to CD120a (TNFRI, p55) and CD120b (TNFRII, p75) and probing with specific internal oligonucleotides, suggested that DC express the CD120b but little if any CD120a. These results were confirmed using monoclonal antibodies to the TNF receptors. Polyclonal antiserum specific for TNF alpha blocked the blood DC-stimulated allogeneic mixed leucocyte reaction (MLR). The addition of TNF alpha to suboptimal MLRs (limited DC stimulators), increased the proliferation of responding T lymphocytes. Having confirmed that T lymphocytes produce TNF alpha and express CD120b after stimulation, we sought to clarify whether the contributing effect of TNF alpha to the allogeneic MLR resulted from a TNF alpha-mediated signal stimulating DC activity, or as a result of autocrine stimulation of T lymphocytes. Pre-incubation of DC with TNF alpha did not increase DC stimulatory capacity and late addition of anti-TNF serum (up to 72 h) still had a significant inhibitory effect on the MLR. We conclude that TNF alpha is probably not involved in the initial DC-T lymphocyte interaction, but acts as an autocrine growth factor for DC induced T lymphocyte proliferation.