The effect of interferon (IFN) and tumour necrosis factor (TNF), either alone or combined with hyperthermia, on cell proliferation and expression of idiotype antigen on a murine B-cell lymphoma has been studied. Incubation with same doses of IFN-alpha and IFN-gamma reduced cell proliferation to the same extent. Hyperthermia potentiated the antiproliferative activity of IFN-alpha and IFN-gamma. Pretreatment with IFN-gamma induced a synergistic response with heat, while IFN-alpha and heat had an additive effect. Tumour necrosis factor (TNF) alone did not affect cell proliferation, nor did TNF modify the heat-induced delay in cell growth. The quantitative expression of surface idiotype antigen was studied by flow cytometry using an anti-idiotype monoclonal antibody (MAb). Heat reduced the expression of idiotype antigen approximately 50%. The duration of the reduction depended on the heat-dose. Recovery of antigen expression correlated with recovery of cell growth, and 2-5 days after the treatment antigen expression returned to the normal level for untreated cells. IFN-gamma and TNF increased antigen expression (30-50%) which lasted for 4-6 days after treatment. When cells were incubated with IFN-gamma or TNF for 2 days prior to hyperthermia, the increase in antigen expression was observed immediately after heating, but by the following day, antigen expression was similar to that after heat treatment alone. Expression of idiotype antigen recovered within 2-5 days to the same values as after heat treatment alone. IFN-alpha alone or combined with hyperthermia did not have any significant effect on antigen expression.