Serum from 39 of 288 (13.5%) cancer patients contained antibodies detectable by immunofluorescence which reacted with frozen tumour sections from kidneys, prostate glands and ovaries. The serum did not react against areas of normal kidney taken from the same diseased organ. In comparison, 0.7% of normal persons and 3.8% of patients with non-malignant diseases had antibody activity. Sections from two colon carcinomas did not react with the serum. It is concluded that tumour-associated antibodies do not necessarily have histological type specificity and are worthy of further study.