The effect of cholic acid pool expansion on biliary lipid composition was investigated in 21 subjects with cholesterol gallstones. All subjects were fed cholic acid (15 mg/kg/day) and ampicillin (2 g/day) in order to depress the intestinal degradation of cholic to deoxycholic acid. Five additional subjects were given ampicillin alone. The treatment lasted 2-3 wk. Parameters investigated included biliary and plasma lipid, biliary bile acid composition, and total and individual bile acid pool size. In 11 of 21 subjects (group I) cholic acid pool expanded two-threefold, whereas deoxycholic acid pool increased only slightly. In this group mean saturation index fell from 1.32 +/- 0.27 (mean +/- SD) to 0.93 +/- 0.25 (p less than 0.001), and plasma cholesterol increased from 185 +/- 45 mg/dl before to 213 +/- 37 after treatment (p less than 0.01). In the remaining 10 subjects (group II) the increase of the deoxycholic acid pool for exceeded that of cholic acid, and in these subjects the saturation index rose from a mean value of 1.07 +/- 0.27 to 1.42 +/- 0.22 after treatment (p less than 0.01). In this group plasma cholesterol tended to decrease (from 213 +/- 57 to 197 +/- 51 after treatment). In the 5 subjects treated with ampicillin alone deoxycholic acid pool was greatly reduced, and the saturation index fell from 1.25 +/- 0.25 to 0.95 +/- 0.35. The results suggest that cholic acid pool expansion reduces bile cholesterol saturation, whereas the increase of deoxycholic acid pool tends to supersaturate the bile. It is concluded that a determinant of bile cholesterol saturation might be the detergent power of the bile acid pool.