The expression of a gastrointestinal carcinoma-associated antigen (GICA), a monosialoganglioside, was investigated in tissues from human fetuses of various gestational ages (10-40 weeks). Mouse monoclonal antibody NS-19-9, generated in mice immunized with SW1116 human colon carcinoma cells, was used along with a second polyclonal antibody to mouse IgG to detect antigen expression as visualized by means of the biotin-avidin-peroxidase assay. Sections of snap-frozen tissues or tissues fixed in 4% formaldehyde, in mercury chloride-formaldehyde, or in Bouin's solution were used. GICA was consistently detected in the mucosal epithelium of the small intestine. In contrast, the mucosal epithelium of the colon-rectum contained no detectable GICA, nor could the antigen be detected biochemically in extracts from the large intestine. GICA was usually found in the epithelium of the larynx, trachea and main bronchi as well as in the conjunctiva, lacrimal and salivary glands, gall bladder and ductus choledochus, and in the epithelium of the renal pelvis of early and mid-gestation fetuses.