Testing of delayed hypersensitivity responses to recall antigens, newly encountered antigens and tumor antigens has contributed to the understanding of several immunologic factors in human neoplasia. Patients with Hodgkin's disease tend to have depressed responses to both newly encountered and recall antigens. Patients with solid tumors are more likely to be deficient only in the response to newly encountered antigens. In patients who have intact response to recall antigens, reactivity to antigen preparations from tumor and control tissue may be studied. Tumor-associated or organ-associated antigens have been demonstrated by delayed hypersensitivity responses in leukemia, Burkitt's lymphoma, malignant melanoma and carcinoma of the lung, breast, cervix uteri and intestine. Approaches to a definition of the specificity of these reactions are described. The results with these tumor antigen tests correlate strongly with the clinical course. This is a promising technique for monitoring immunotherapy. The results from tests with recall and newly encountered antigens also correlate with the clinical status and perhaps with prognosis. Various possible interpretations of these changes are discussed. Further work should be directed toward an exact definition of immunologic defects in patients with cancer and toward the use of this understanding for a rational program of immunotherapy.