The effect of ingestion of 50 g fat, 50 g protein, and 50 g starch on plasma cholecystokinin (CCK) concentrations was studied in eight healthy volunteers. Plasma CCK concentrations were measured by radioimmunoassay with Bolton-Hunter-labelled CCK 33, CCK 33 standard, and antibody T204. Antibody T204 was directed to the sulphated tyrosine region of CCK. Ingestion of fat and protein induced significant increases in plasma CCK, whereas ingestion of starch did not significantly influence plasma CCK levels. Peak increments in plasma CCK after fat (4.8 +/- 0.9 pmol/l) and protein (3.4 +/- 0.5 pmol/l) were significantly greater than that after starch (0.9 +/- 0.3 pmol/l). Similarly, the integrated plasma CCK secretion after fat (213 +/- 49 pmol/l X 120 min) and after protein (178 +/- 53 pmol/l X 120 min) was significantly greater than that found after ingestion of starch (9 +/- 23 pmol/l X 120 min). It is concluded that, in contrast to starch, fat and protein are potent stimuli for the release of CCK.