Radioimmunoassay for secretin

: A simple direct radioimmunoassay for human secretin has been developed by immunizing rabbits with synthetic human secretin. After 1-2 injections antisera with titers up to 1:52000 could be induced. In a dysequilibrium system secretin levels in plasma can be measured down to a concentration of 25 pg/ml plasma. Extraction methods gave unreproducible recoveries. Direct estimation in plasma provides only relative values because of a "non-specific" effect of secretin free plasma which is difficult to control. A simple assay based on iodination with chloramin-T and charcoal dextran separation produces at least reliable relative values, as shown by the pattern of secretin concentrations in response to intraduodenal stimulation with HC1.