Mice bearing progressively growing syngeneic methylcholanthrene-induced sarcomas are immunologically hyporeactive. However, both basal (steady-state) and bacterial lipopolysaccharide (LPS)-induced synthesis of mRNA for interleukin-1 (IL-1) in peritoneal exudate cells (PEC) or spleen cells were comparable in control and tumour-bearing animals. Furthermore, the production of IL-1 by PEC stimulated with LPS in the presence of indomethacin was same in control and tumour-bearing mice. The results thus demonstrate that LPS-stimulated cells from animals bearing progressively growing syngeneic sarcomas synthesise the same quantities of mRNA for IL-1 and produce comparable amounts of IL-1 as do cells from normal animals, in spite of the profound immunological hyporeactivity of the former.