The response of the local secretory immunoglobulin system to tumor antigens was studied in 71 patients suspected of harboring a gastric malignancy. Gastric aspirates were obtained by fiberoptic endoscopy and measured on single radial immunodiffusion plates (Behring Diagnostics). Patients with malignant neoplasms of the stomach were divided into Type I and Type II lesions, based on the extent and stage of the disease. Secretory IgA titers were elevated in all patients with a Type I malignant tumor confirmed by histologic examination; mean 43.8 mg/100 (SD ± 40.6 mg/100). Patients with Type II malignant tumors, benign lesions or normal controls had significantly lower IgA titers; mean 6.1 mg/100 (SD ± 3.7 mg/100). This leads us to believe that the host's response to malignant cells of the gastrointestinal system is an altered and elevated secretion of IgA. Evaluation of local immunoglobulin titers may be of value when used in conjunction with other diagnostic modalities in determining the nature of gastrointestinal lesions, particularly when the lesion is not metastatic or massively involving the gastric wall.