Surfactant proteins SP-B (M r = 8700, reduced) and SP-C (M r = 3000-6000, major form, non-reduced) interact with surfactant phospholipids to enhance their surface active properties. In the present study, we describe the structural and functional characteristics of a novel dimeric form of bovine SP-C (M r = 9000, non-reduced), which is identified as [SP-C]2. Dimeric SP-C exhibits surface tension-lowering properties differing from those of monomeric SP-C and enhances the surface properties of bovine SP-B/phospholipid mixtures. Chemical analysis indicated that [SP-C] 2 was not acylated at the cysteinyl residues. Fourier transform-infrared spectroscopy (FT-IR) was utilized to determine the secondary structures of [SP-C] 2 in DPPC films. Relative percentages of a-helical,/3sheet, B-turn and random coil structures were calculated by peak fit analysis of the amide I band of the FT-IR spectra indicating that, in contrast to the helical structure of monomeric SP-C, [SP-C]2 exhibits almost exclusively B-sheet structure. In addition, only i 0% of the amide (backbone) hydrogens exchanged with deuterium of I)2 O, indicating that the remaining 90% of amide hydrogens were not accessible to D20 due to strong hydrogen bonding or their location in a hydrophobic environment. Dimerization of SP-C effects a major change in secondary structure, a factor which may play a role in the interaction of SP-C with phospbolipids in pulmonary surfactant.