Human interleukin-5 is a homodimer; each subunit contains two cysteine residues that form two inter-subunit disulfide bonds. The topology of the disulfides in recombinant human interleukin-5 produced in Escherichia coli was studied by proteolytic digestion and peptide mapping. Disulfide linked peptides containing cysteine 42 linked to cysteine 84 were isolated. This indicated that cysteines 42 and 84 of one subunit were linked in an antiparallel manner to cysteines 84 and 42 of the other subunit.