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We have found that a slightly modified insulin-like growth factor II (IGF II) consisting of a chimaera of bombyxin and human IGF II (BOMIGF) is properly secreted in insect cells by using the baculovirus expression system. Human interleukin 3 (IL-3) was attached to the C-terminal amino acid residue of BOMIGF with peptide linkers containing five or twelve residues. Only the chimaera with the 12-residue linker had biological activities of both IGF II and IL-3. The chimaera had a significantly higher mitogenic activity than IL-3 in cell cultures of the human haemopoietic cell line TF-1 and its effect could be observed even at femtomolar concentrations. It was also able to stimulate thymidine incorporation in IGF II-dependent bovine fetal erythroid cells. The chimaera significantly increased the number of macroscopic haemopoietic colonies in cultures of human peripheral blood in comparison with IL-3 or mixtures of IL-3 and BOMIGF in vitro. Subcutaneous injection of a BOMIGF-mouse IL-3 chimaera in normal C57BL/6 mice resulted in a significant increase of the number of spleen stem cells producing macroscopic haemopoietic colonies. This new system for the biosynthesis of IGF-cytokine fusion proteins in insect cells might prove advantageous for the low-cost and high-yield production of molecules with complementary or synergistic biological activities.