A cDNA fragment coding human interleukin-6 (IL-6) was inserted into the vector pSXIVVI(+)X3 with the control of Syn and XIV promoters. The Sf9 cells (Spodoptera frugiperda) were co-transfected with the recombinant plasmid and TnNPV DNA (Trichoplusia ni nuclear polyhedrosis virus DNA). Cells infected with recombinant virus synthesized IL-6 protein at a level of approx. 42% of total cellular protein. IL-6 activity in infected cells was measured by using the IL-6-dependent mouse B hybridoma cell line 7TD1 and MTT colorimetric assay, the highest expression level, 2.82 x 10(6) u/ml, was obtained at 96 h after infection. Western blot analysis of infected larvae protein extracts showed that the virus-mediated IL-6, with a molecular weight of similar to 21 kDa, shared antigenic sites with the authentic counterpart. However, no evidence of its bioactivity was found, IL-6 activity was determined only for IL-6 expressed by insect cells.