A squamous cell carcinoma of spontaneous orgin in a WHT/Ht mouse was used to study the frequency with which the regional axillary lymph nodes draining subcutaneous or intradermal tumours gave rise to tumours after their isogeneic transplantation as whole nodes. This frequency (similar to 40%) was found not to vary significantly with the size or duration of the tumour drained and not to be increased by coincident infective, traumatic or antigenic stimuli acting at the tumour site or in adjacent tissue. Because tumour growth occurred in only 2/55 (4%) nodes which were left in situ in mice whose tumours were radically excised, it was concluded that tumour forming node transplants reflected a small and limited content (estimated to be about 13) of transnodally passing tumour cells destined to pass on to the blood; separate experiments showed that tumour cells reaching the blood survived for only a few hours. Nodes from tumour-excised mice gave rise to tumours as frequently when autografted as when isografted to mice with no previous expose to the tumour. A review of the finding reported here and of previous quantitative data for this system enabled us to exclude any implication of anti-tumour immunity from our interpretation of the results of the experiments.