We measured gallbladder mucin production by hamsters fed diets lithogenic for either cholesterol or pigment gallstones. In hamsters on the cholesterol stone diet, gallbladder production of 3H-glucosamine-labeled mucin was elevated two- and seven-fold after 1 and 3 weeks, respectively. After 1 week cholesterol crystals were seen in a mucus gel on the gallbladder surface. In hamsters on the pigment stone diet, gallbladder mucin production was significantly elevated after 1 and 3 weeks. The first precipitation of pigment crystals was in mucus in bile or on the gallbladder surface. Black pigment stones grew by agglomeration of pigment crystals enmeshed in mucus. In conclusion, gallbladder mucin production is increased before cholesterol or pigment stone formation, and the earliest deposition of crystals is in mucus in bile or on the gallbladder surface.