To gain insight into the regulation of mucin biosynthesis and secretion at a molecular level, we studied the expression of four mucin genes in several secretory cell types. Oligonucleotide probes for MUC1, MUC2, MUC3, and MUC4 were used in Northern blot analysis of total RNA from the cells. It was shown that MUC4 was constitutively expressed in the endometrial cell line (Ishikawa), which we use as a model system for studying airway mucin. Expression was enhanced by secretagogues. Cultured bronchial epithelial cells gave identical results. MUC1, MUC2, and MUC3 were not detected in either cell system. Secretory phase endometrial tissues also expressed only MUC4, but proliferative tissues expressed both MUC4 and MUC1. The data indicate that the mucin genes are differentially expressed in various cells and suggest a possible regulatory role for steroid hormones.