Twenty-five gallbladders were studied in vitro. Sixteen had radiolucent gallstones and 9 had radiopaque gallstones. The radiolucent gallstones had a cholesterol content of 94.17 ± 3.76% and the radiopaque gallstones had a cholesterol content of 56.6 ± 4.46%. Half the maximal response (ED50) to cholecystokinin octapeptide (CCK-OP) and to carbachol in strips from patients with radiolucent gallstones was 0.8 ± 0.15 and 27.01 ± 3.74 × 10-7M, respectively. In strips from patients with radiopaque gallstones, the ED50 was 0.4 ± 0.08 and 14.92 ± 3.07 × 10-7M, respectively. The ED50 values to CCK-OP and carbachol were greater in strips from specimens with radiolucent gallstones than in strips from specimens with radiopaque gallstones (p < 0.05). There was no significant difference in the maximal contractile response of the two groups. It can be concluded that gallbladder sensitivity to CCK-OP and carbachol can be modified in relation to differences in the cholesterol and calcium content of the