A fluorometric immunoassay has been established to quantitate pancreatic stone protein providing a sensitivity for concentrations from 0.015 to 0.5 micrograms/mL. When concentrations of pancreatic stone protein were determined from pancreatic secretions obtained either from patients suffering from chronic pancreatitis (n = 31) [including the calcifying forms (n = 10)], pancreatic cancer (n = 22), or nonpancreatic diseases (n = 17), no significant differences were found. In contrast, increased concentrations were found in serum samples from patients with chronic (39/66) and acute pancreatitis (16/20) compared with control patients. The differences between these diagnostic groups and controls were highly significant (P less than 0.0001) and independent of pancreatic enzyme activity. Immunochemical analyses of serum pancreatic stone protein showed an isoelectric point (pH 9) similar to that reported for the pancreatic thread protein. With respect to recent communications, these data do not support the etiopathogenic role postulated for pancreatic stone protein in chronic pancreatitis and chronic calcifying pancreatitis by other investigators.