Patients with progressing tumours have circulating leucocytes which are sensitised to tumour specific antigens and their serum contains blocking factors. Patients who have tumours which have been successfully destroyed do not have circulating blocking factors although their lymphocytes continue to remain sensitised to the tumour antigens. Two tests, known to detect cellular sensitisation and serum blocking activity, the leucocyte adherence inhibition test (LAI) and the macrophage migration inhibition test (MMI) have been compared in a small group of patients with malignant melanoma and carcinoma of the colon. There was agreement between the results of both tests in over three-quarters of the tests performed and in those which failed to agree neither test was favoured in relation to the known tumour state of the patient. Thirty-two tests were performed in 22 patients, there was one false positive with MMI testing, seven false negative results for the LAI test and six for the MMI test.