Mature lung surfactant protein C (SP-C) corresponds to residues 24–58 of the 21 kDa proSP-C. A late processing intermediate, SP-C i, corresponding to residues 12–58 of proSP-C, lacks the surface activity of SP-C, and the SP-Ci a-helical structure does not unfold in contrast to the metastable nature of the SP-C helix.