The mucin gene MUC1, is expressed in a number of human ductal epithelia in vivo including those within the pancreas, mammary gland, kidney and genital ducts. Further it is expressed at a high level in certain tumours and tumour-derived cell lines. MUC2 was initially isolated from a human jejunum cDNA library and is thought to be one of the major intestinal mucin genes, though it is also expressed in the trachea. We have examined the developmental expression of these two mucin genes in human tissues. High level expression of MUC1 has been seen by 12.5 weeks of gestation in the epithelial of the distal respiratory tract and the collecting ducts in the kidney. By 18 weeks MUC1 mRNA is detectable in the colon but pancreatic expression of MUC1 is not seen until late in gestation. MUC2 mRNA is seen by 12 weeks of gestation in the jejunum, ileum and colon, and in large bronchioles of the lung by 18 weeks. The pattern of expression of MUC1 suggests that this mucin may not be involved in early ductal obstruction in the CF pancreas, but both MUC1 and MUC2 may play a role in the development of intestinal disease and MUC1 in early respiratory disease associated with CF.