Fluorescein-dilaurate has been recommended as a screening test for the assessment of exocrine pancreatic function. In this test fluorescein excretion in the urine is measured for an 8--10 hour period. To avoid urine collection the present study reports on serum fluorescein determinations following oral administration of fluorescein-dilaurate-ester. According to the results of a standard secretin-pancreozymin-test 24 subjects were classified as 10 persons with normal pancreatic function, 7 patients with mild and 7 patients with severe exocrine pancreatic insufficiency. Fluorescein concentrations were measured photometrically after TCA-precipitation. In the normal persons maximal fluorescein levels were observed 4--6 hours after oral load of the test substance. Patients with mild pancreatic insufficiency showed the same pattern and only in patients with severe pancreatic insufficiency much lower serum fluorescein concentrations were found. They were statistically significantly different from normal subjects 3--6 hours after administration of fluorescein-dilaurate.