An in vitro bioassay for cholecystokinin which is superior to the previously described in vivo assays has been properly validated mathematically. The specificity of the assay has been assessed by measuring the potency of other polypeptides which share the same C-terminal pentapeptide sequence with cholecystokinin (gastrin, caerulin, octapeptide of cholecystokinin, and pentagastrin). The assay is shown to be quite specific for cholecystokinin. Secretin does not interfere with the assay. Comparison of the relative potency of the two commercially available preparations of cholecystokinin indicates that the two major units of measurement are almost identical in cholecystokinetic potency. One Crick Harper Raper unit of cholecystokinin (Boots) is equivalent to 1.22 +/- 0.12 Ivy dog units of cholecystokinin (Karolinska Institute). The lower limit of sensitivity for the assay was 2.5m IDU/ml.
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