A direct sandwich immunoassay was developed to quantify the human reg protein, a non enzymatic pancreatic acinar protein the biological function of which remains elusive. Polystyrene balls were coated with specific IgG fraction as the first antibody and horseradish peroxidase labelled IgG was used as a second antibody. The linearity of the assay was good over a concentration range of 1.25-100 ng/ml and the good parallelism obtained between the standard and the assay dilution curves in serum and pancreatic juice indicates the absence of non-specific interfering reactions. Gel filtration of serum showed that the reg protein was eluted in the fractions corresponding to the proteins of around 25 kDa and that the chromatographic behaviour of the serum protein was identical to that of the purified pancreatic protein when added to serum. This assay was simple, specific, sensitive and reproducible and may permit the determination of low levels of reg protein in different biological fluids.