The projected areas of nuclei of tumour cells in experimental and human tumours have been measured with a Quantimet. In nearly all the specimens examined the nuclei in the lymph node metastasis, and the nuclei in the site from which metastasis had occurred, were similar in size. In one case of human breast cancer, nuclei in the axillary nodal metastasis were twice as large as those in the mammary primary. In another case of human breast cancer a few very large nuclei were seen only in the mammary primary.