Our previous studies have demonstrated the presence of two of the three vertebrate inflammatory cytokines in invertebrates, namely, interleukin (IL)-1 and tumour necrosis factor (TNF). We studied the coelomic fluid of the echinoderm, Asterias forbesi, to determine whether it contained the third inflammatory cytokine, IL-6. Coelomic fluids were concentrated and then fractionated by a combination of gel sieve chromatography and preparative isoelectric focusing. The M, observed for the invertebrate IL-6-like molecule was approximately 30,000 and the pI was 5.5. Western blot analysis using a polyclonal rabbit antiserum to human IL-6 detected a band at approximately 30,000 Da. The invertebrate IL-6 was inhibited by an antiserum to human IL-6 when used in the B9 assay. Finally, the coelomocytes were found to be capable or releasing the IL-6-like molecule as early as 12 h following stimulation by LPS. These results, together with our other data, show that echinoderms possess correlates of all three vertebrate inflammatory cytokines.