Plasmid vectors that carry complementary DNA (cDNA) clones coding for polypeptides exhibiting human granulocyte/macrophage colony stimulating factor activity, including eosinophil growth factor activity. One of these polypeptides is about 144 amino acids in length, including a potential leader sequence. The cDNA is derived from messenger RNA isolated from a human cell source after activation with concanavalin A. The cDNA was cloned by incorporation into a plasmid vector, which was then transformed into E. coli. The plasmid vector also contained DNA segments from the SV40 virus, permitting expression of the cDNA after transfection into a mammalian host cell, such as monkey COS-7 cells.