In vitro cell‐mediated immunity was assayed by leukocyte adherence inhibition (LAI) to determine the extent of autosensitization to myelin basic protein (MBP). Leukocytes from 123 cancer patients, 16 patients freed of cancer, 135 patients with benign disease, and 26 patients with destruction of nervous parenchyma were tested. Most patients with cancer reacted to MBP: 92%, 93%, 82%, 78%, 75% and 62% for pancreatic, colonic, esophageal, lung, ovarian and breast. Few patients with benign diseases reacted to MBP. Patients with multiple sclerosis (MS) were sensitized to MBP, but patients with other nervous tissue injury did not react to MBP. Cancer patients did not remain sensitized to MBP once they were freed of their cancer. The LAI assay is a straightforward method of measuring cellular autosensitivity to MBP. In the population of patients tested, autosensitivity to MBP was confined, except for MS, principally to cancer patients.