Abstract
Characteristics of cells that are associated with the hormonal dependence of tumours are described, and it is shown that clonogenicity and hormone-induced proliferative response of breast tumours are as good markers of hormonal dependence as is oestrogen receptor. Thus tumours that formed less than 150 colonies per 500,000 cells seeded and that increased their proliferative activity 1.8-fold or more in response to hormones were the tumours that were likely to respond to endocrine treatments, whereas all other tumours were likely to be refractory to endocrine treatments. These two criteria (clonogenicity and proliferative response to growth hormones) correctly identified the response to subsequent endocrine treatments in 15 out of 17 patients with oestrogen receptor-unknown tumours. It is proposed that they may constitute a substitute for the oestrogen receptor status in patients with non-biopsiable tumours, and an additional discriminant where the oestrogen receptor assay is available.