Surgical specimens from 9 normal human gallbladders and from 9 lithiasic gallbladders were incubated in vitro with [3H]thymidine. Autoradiography was used to observe the cells in DNA synthesis in the mucosal epithelium. The percentages of labeled cells (labeling index) and of mitotic figures (mitotic index) were estimated. In the normal gallbladder, the labeling index was low and mitotic figures were exceptional; the labeled cells in the epithelium were observed against the basal membrane. In the lithiasic gallbladder, the labeling index was 23 times higher (P less than 0.01) and a definite number of mitotic figures was seen; the uptake of [3H]thymidine was not restricted to basal epithelial cells and occurred at any level of the mucosa. Our data showed that the proliferative activity was increased in the mucosa of the human lithiasic gallbladder. To the extent that the labelled cells against the basal membrane in the normal gallbladder represent the epithelial progenitors, our results also indicated modifications in the spatial distribution of labeled cells in the lithiasic gallbladder mucosa. Such concomitant quantitative and qualitative changes in cell kinetics could have a parallel in known precancerous conditions in the digestive tract.