Electrospray (ES) mass spectrometry has been used to analyse preparations of porcine pulmonary surfactant polypeptide-C (SP-C). A number of variant forms of the native 35-residue dipalmitoylated peptide were detected including (a) C-terminally methylated SP-C, (b) C-terminally methylated and methionine oxidized SP-C, (c) N-terminally truncated, C-terminally methylated and methionine oxidized SP-c, (d) C-terminally elongated, C-terminally methylated and methionine oxidized SP-C, and (e) tripalmitoylated, C-terminally methylated and methionine oxidized SP-C. C-terminal methylation and methionine oxidation are probably a consequence of the sample handling procedure. The occurrence of the C-terminally elongated form of SP-C has implications for the in vivo processing of proSP-C and the Tandem mass spectrometry (MS/MS) was used to confirm the amino acid sequence of SP-C and the presence of palmitoyl groups covalently linked to the peptide. Some of the structures of the variant forms of SP-C were determined by MS/MS.