One hundred and twenty-nine canine mammary tumours of varying histological types were examined for the presence of oestrogen binding protein by the dextran-coated charcoal method with the binding parameters of the reaction determined by Scatchard plots. By this method, 39 per cent of the 129 mammary tumours were shown to contain oestrogen receptor protein. This figure fell to 25 per cent for benign tumours and rose to 52 per cent for malignant tumours. There was no correlation between the histological nature of tumours and oestrogen receptor content in either benign or malignant forms. Oestrogen receptors were not demonstrable in normal mammary tissues. Whether the presence of oestrogen receptors in a tumour is indicative of hormonal dependence as is reported for human breast tumours is being investigated in the bitch.

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Twenty primary or metastatic felline mammary tumours were assayed for the presence of oestrogen receptors by the dextran-coated charcoal method. In only two of the tumours was the presence of specific oestrogen binding protein demonstrable. The high negative rate in comparison with human and canine mammary tumours assayed under similar conditions seemed to be a peculiar feature of the feline tumours and suggested a lack of oestrogen dependency. The possibility remains that other hormones may be concerned with the growth of feline mammary tumours and further work is in progress.

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