Using a binding assay for sex hormone binding globulin, a Japanese group showed that they could select breast cancer patients more likely to be responsive to endocrine therapy, i.e. oestrogen receptor positive patients, by an increase in oestrogen binding capacity of the protein. We directly measured the levels of this protein using a specific antiserum to it, and the levels were not different from normal in any group of breast cancer patients whether or not oestrogen receptors were present. Both methods of measurement can detect a slight increase after a weak oestrogenic stimulus, indicating that they are as sensitive as each other.