Single Light Green Cells (LGC) of Lymnaea stagnalis, expressing four genes encoding insulin-related peptides (MIPs) and C-peptides, and sections from the median lip nerve (MLN) were subjected to MALDI-MS. Mass spectra of LGCs and MLNs were almost identical. Masses corresponding to those of the MIPs and some C alpha-peptides could be distinguished. ProMIP III C alpha-peptide and C beta-peptides were not found. The spectra showed additional masses matching those of carboxyterminally truncated C alpha-peptides. Peptides with similar masses were isolated from MLN extracts by HPLC, using electrospray-MS screening. Amino acid sequence analysis revealed intact proMIP I, II and V C alpha-peptides and I, II C alpha-peptide 1-24, 1-22 and 1-15.