Abstract
1. Successive sampling of ovarian cancers during cytostatic treatment showed several cases of notable changes in their ploidy distribution and one change in model chromosome number, indicating selection of a resistant tumour-cell population. 2. Studies of cell suspensions from human tumour specimens incubated with [3H]-TdR after exposure in vitro to various cytostatic agents have shown variation in labelling between different parts of the same tumour, as well as between the primary tumour and its metastases or ascitic tumour-cell population, which may be accounted for by variation in sensitivity of the tumour-cell population. 3. Studies of nuclear morphology in 20 endometrial cancers before and after progesterone therapy demonstrate considerable variation in the proportion of cells undergoing secretory conversion within the same tumour, indicating primary heterogeneity of the tumour-cell population in response to progesterone.