1. The effect of the R3230 AC rat mammary adenocarcinoma on progesterone binding in the liver of female F344 rats has been studied. The growth of the transplanted tumour shows a positive correlation with increased binding of progesterone to the endoplasmic reticulum.

2. Both total binding rate and saturation level in the formation of specific progesterone microsomal receptor complex are increased.

3. This is paralleled by an elevated progesterone content of microsomal membranes.

4. in contrast, nonspecific progesterone binding in the microsomes is reduced. These results may represent a paraneoplastic change.

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