The protein composition of normal human pancreatic juice and pancreatic juice of men with chronic calcifying pancreatitis (CCP) were compared using immuno-diffusion techniques. An additional protein was found in pathological pancreatic juice and this protein has been identified as lactoferrin. Its concentration has been determined. The indirect immunofluorescence method was used to detect and localize lactoferrin in 9 samples of human pancreas from 3 patients with CCP and 6 patients without CCP. The fluorescence was found in all samples. Lactoferrin staining was localized in lumen acinar and in the cytoplasm of the acinar cells. The fluorescence was stronger in the apical zone of acinar cells. When lactoferrin antiserum was incubated with lactoferrin insolubilized by glutaraldehyde, no fluorescence staining was detected. The differences between our results obtained by immunodiffusion and immunofluorescence techniques are explained by the different sensitivity of the two techniques and these results indicate that lactoferrin is a protein of pancreatic secretion.