Sixty cases of colonic carcinoma have been investigated for antitumour immunoreactivity. The tests employed were blood lymphocyte reactivity and complement-dependent serum cytotoxicity against cultured tumour cells, and immunofluorescence for membrane staining of viable tumour cells and cytoplasmic staining of dried tumour cells in films. Nineteen cases were positive by one or more tests and the most frequent positive response, lymphocytotoxicity, was detected in 8 of the 24 cases tested in this way. The lymphocytotoxicity persisted in a case tested three times over a year. Immunoreactivity against tumour cell surface, as by lymphocyte or serum cytotoxicity or membrane immunofluorescence, was restricted to colonic carcinomas but there was an additional element of individual specificity; cross-reactivity with other tissues and tumours was not observed. Lymphocytes from regional lymph nodes were non-reactive even in a case with positive blood lymphocyte cytotoxicity against the carcinoma cells.