The inhibitory activity of gallbladder bile on in vitro calcium carbonate crystallization was investigated in 12 normal subjects, 18 patients with radiolucent gallstones, and 18 patients with radiopaque or mixed gallstones, all of whom had functioning gallbladders. Inhibitory activity, expressed as CIT (crystallization inhibition time), was notably higher in the normal subjects and only slightly greater than the effect of water in the gallstone patients; the extrapolation of the results suggests that the difference in the method used to obtain the bile, by duodenal probe or during surgery, had little effect on the inhibitory activity. No significant correlation was found with the CSI (cholesterol saturation index) and the biliary lipid variables. In view of the frequent appearance of calcium carbonate in the composition of gallstones, the possibility is emphasized that the reduced inhibitory effect of bile in the crystallisation of CaCO3 plays a part in gallstone growth or formation.