Coculture of paraformaldehyde-fixed macrophages with syngeneic thymocytes resulted in synthesis of a lymphokine which triggered IL-1 production in untreated, peritoneal macrophages. The release of that lymphokine was MHC restricted, however, the stimulation of IL-1 production was not MHC-controlled event. Preliminary studies showed that the lymphokine had molecular weight (MW) of around 10-30 kDa, was heat stable (56 degrees C), resistant to reduction to DTT and exhibited a weak pyrogenic activity. Fusion of activated thymocytes with the thymoma BW5147 resulted in obtaining several hybridoma clones producing the lymphokine. Preparation of culture supernatants by means of Amicon membranes and Sephadex G-100 filtration allowed to determine the MW of the lymphokine as 25 kDa. Lastly, the lymphokine was identified as IL-6 by means of anti-IL-6 antibodies which blocked its function and by the use of IL-6-sensitive cell line 7TD1.