Plasma from a patient with chronic pancreatic pseudocyst showed an additional more negative albumin band (18%) on agarose gel electrophoresis. Both components bound (63)Ni(2+), indicating intact N-terminals; however, electrospray ionisation analysis of the intact proteins showed the mass of more negative albumin was 1254 Da less than the control and that the apparently normal band was 112 Da less. Reverse phase mapping and mass analysis of CNBr peptides showed three proteolytically modified forms of the C-terminal peptide indicating that some 81% of the albumin molecules lacked the C-terminal Leu residue, that 18% lacked the C-terminal KKLVAASQAALGL and that approximately 1% lacked the QAALGL sequence. These findings were further verified by tryptic mapping of the aberrant CNBr peptides. The truncations probably result from exposure of the albumin to 'leaking' pancreatic endo and exoproteases. During less acute phases of the disease, the 13 and 6 residue truncated forms together decreased to less than 1%, while the des-Leu(585) form made up the balance; no normal albumin was detected. This suggested that the des-Leu(585) form might be present at low levels in the plasma of normal individuals and CNBr mapping confirmed that it constituted 4-15% of the albumin from normal plasma.