A series of three aromatic to alanine mutants of recombinant murine interleukin-6 lacking the 22 N-terminal residues (ΔN22mIL-6) were constructed to investigate the role of these residues in the structure and function of mIL-6. While Y78A and Y97A have activities similar to that of ΔN22mIL-6, F173A lacks biological activity. F173A retains high levels of secondary structure, as determined by farUV circular dichroism (CD), but has substantially reduced levels of tertiary structure, as determined by near-UV CD and 1H NMR spectroscopy. F173A also binds the hydrophobic dye 1-anilino- 8-naphthalenesulfonic acid (ANS) over a range of pH values and exhibits noncooperative equilibrium unfolding (as judged by the noncoincidence of monophasic unfolding transitions monitored by far-UV CD and λ(max), with midpoints of unfolding at 2.6 ± 0.1 and 3.5 ± 0.3 M urea, respectively, and the lack of an observable thermal unfolding transition). These are all properties of molten globule states, suggesting that the loss of activity of F173A results from the disruption of the fine structure of the protein, rather than from the loss of a side chain that is important for ligand- receptor interactions. Surprisingly, under some conditions, this loosened conformation is no more susceptible to proteolytic attack than the parent protein. By analogy with human IL-6, Phe173 in ΔN22mIL-6 makes multiple interhelical interactions, the removal of which appear to be sufficient to induce a molten globule-like conformation.