Hepatocyte-stimulating factor, interferon-beta 2, B-cell stimulation factor 2, and hybridoma/plasmacytoma growth factor are identical proteins presently referred to as interleukin 6 (IL-6). Through the use of synthetic oligonucleotide technology, we have constructed a biologically active recombinant IL-6 (rIL-6) gene based on the sequence of a human IL-6 cDNA. The synthetic gene encodes a cysteine-free, bioengineered rIL-6 protein that is expressed at high levels in Escherichia coli as a tripartite fusion protein. Cleavage of the fusion protein with collagenase releases a 23-kDa rIL-6 protein that can be easily purified to homogeneity. We show that the rIL-6 protein displays a range of biological activities similar to those of natural human IL-6, as demonstrated by its ability to (i) protect cells from viral infection, (ii) stimulate the synthesis of fibrinogen in rat FAZA 967 cells, and (iii) induce the terminal differentiation of B cells, resulting in elevated secretion of immunoglobulin.