mGM-CSF fusion proteins are bioactive as measured by growth of GM-CSF–dependent NFS-60 cells. NSF-60 cells were grown in the presence of varying concentrations of 38C13 idiotype fusion proteins prepared in the cell-free expression system or commercial murine GM-CSF (mGM-CSF) standard. All dilutions were performed in triplicate. Neither the human fusion (hGM-Im9-scFv) nor scFv alone induced cell growth, but the fusion protein containing mGM-CSF fused to scFv promoted the growth of NFS-60 cells with the same potency as a commercial standard.