Localization and intensity of squamous cell carcinoma-related antigen (SCC-Ag) and infiltration of Langerhans' cells (LC) were investigated immunohistochemically, using anti-SCC-Ag and anti-S-100 protein antisera, respectively, in 36 cases of squamous cell carcinoma of the human esophagus. The SCC-Ag was an indicator of histologic differentiation of squamous cell carcinoma but not of malignant potential. The population density of LC correlated well with the histologic stage, vessel invasion, and postoperative survival time. In patients with a marked infiltration of LC, survival time was longer than for those in whom the infiltration was slight (P less than 0.05). The density of the LC can serve an indicator of defense of the host against the carcinoma. There was no correlation between SCC-Ag and S-100 protein-positive cells.