The relationship among protein oligomerization, secondary structure at the interface, and the interfacial behavior was investigated for spread layers of native pulmonary surfactant associated proteins B and C. SP-B and SP-C were isolated either from butanol or chloroform/methanol lipid extracts that were obtained from sheep lung washings. The proteins were separated from other components by gel exclusion chromatography or by high performance liquid chromatography. SDS gel electrophoresis data indicate that the SP-B samples obtained using different solvents showed different oligomerization states of the protein. The CD and FTIR spectra of SP-B isolated from all extracts were consistent with a secondary structure dominated by α-helix. The CD and FTIR spectra of the first SP-C corresponded to an α-helical secondary structure and the spectra of the second SP-C corresponded to a mixture of α-helical and β-sheet conformation. In contrast, the spectra of the third SP-C corresponded to antiparallel β-sheets. The interfacial behavior was characterized by surface pressure/area (π-A) isotherms. Differences in the oligomerization state of SP-B as well as in the secondary structure of SP-C all produce significant differences in the surface pressure/area isotherms. The molecular cross sections determined from the π-A isotherms and from dynamic cycling experiments were 6 nm2/dimer molecule for SP-B and 1.15 nm2/molecule for SP-C in α-helical conformation and 1.05 nm2/molecule for SP-C in β-sheet conformation. Both the oligomer ratio of SP-B and the secondary structure of SP-C strongly influence organization and behavior of these proteins in monolayer assemblies. In addition, α-helix → β-sheet conversion of SP-C occurs simply by an increase of the summary protein/lipid concentration in solution.