A partly folded form (I) of apomyoglobin has an alpha-helix content of about 35%; in an earlier study, hydrogen exchange revealed that the A, G, and H helices are folded, while much of the rest of the protein is not [Hughson, F. M., Wright, P. E., & Baldwin, R. L. (1990) Science 249, 1544-1548]. Because A, G, and H form a compact subdomain in native myoglobin, we proposed that nativelike packing interactions among the three helices might be retained in the I form of apomyoglobin. To test this proposal, disruptive mutations were introduced into the A.H and G.H helix packing sites. These mutations destabilize native apomyoglobin relative to I. In contrast, the stability of I is relatively insensitive to mutation; in particular, side-chain volume alone does not appear to be important. These results indicate that the I form is not stabilized by nativelike A.H and G.H packing interactions. In support of this we show that partly helical peptides derived from the G and H helix regions of myoglobin do not pair in solution. Since the isolated G and H peptides are at best only partly helical, some type of interaction must stabilize these helices in the I form. Small increases in the stability of I are seen when mutation introduces a side chain of increased nonpolar surface area. We suggest that I is stabilized by relatively nonspecific hydrophobic interactions that allow it to adapt easily to mutation. In this and other respects, I appears to conform to the "molten globule" model, with the caveat that only part of the polypeptide chain appears to participate in the globule.