Although cholecystokinin-58 (CCK-58) is a major molecular form stored in the intestine, it has not yet been shown to be released into the circulation. This report describes in vitro degradation of CCK-58 in human blood and plasma and the molecular forms detected when this degradation is inhibited. After incubation of CCK-58 for 150 min between 20 and 24 degrees C, approximately 60% of immunoreactivity recovered was degraded to smaller immunoreactive forms. Storage of the 150-min incubate at -20 degrees C for 3 days greatly increased the observed degradation to 85%. When CCK-58 was added in vitro to blood, similar degradation occurred. Degradation of CCK-58 could be inhibited by addition of acid. Blood was obtained 1 h after a test meal designed to stimulate CCK release. The pH was lowered during collection and processing of blood and plasma to inhibit in vitro degradation of cholecystokinin. This method permitted the detection of significant amounts of CCK-58 in circulation.