We have combined the molecular biology methods of the polymerase chain reaction and recombinant DNA cloning in bacteriophage lambda to express a human IgM Fab in Escherichia coli using genes derived from an Epstein-Barr virus transformed cell line. This method comprises three cDNA amplifications and a single cloning step, culminating in the stable overexpression of mammalian heterodimeric recombinant protein in a prokaryotic host.