The regulation of mucin secretion by SW1116 human colon carcinoma cells has been studied using monoclonal antibody 19-9, which has previously been used to detect mucin in the serum of cancer and cystic fibrosis patients. We found that SW1116 cells constitutively secrete considerable amounts of mucin as the predominant glycoprotein. The secretion of mucin by these cells is independent of cyclic AMP levels, but can be further stimulated by the Ca2+ ionophore A23187. However, arachidonic acid and its metabolites inhibit mucin secretion. Electron microscope studies reveal that the mucin is located near the plasma membrane as well as in vesicular and lysosome-like structures. However, the secretion pathway of mucin is different than that of the lysosomal contents, since arachidonic acid, while inhibiting mucin secretion, actually activates the secretion of the lysosomal enzyme beta-glucuronidase. We suggest that the mechanism of mucin secretion by SW1116 cells occurs by a pathway different from common exocytosis, and possibly by more than one pathway. The response of mucin secretion by SW1116 cells to common secretagogues resembles that of epithelial cells obtained from cystic fibrosis patients. Thus SW1116 cells are an especially interesting system for studying processes related to pathological states associated with excessive constitutive secretion of mucin.