'Specific receptor binding' of radioactively labeled products of radiolysis of the estrogen receptor ligand R 2858 (17 alpha-ethynyl-11 beta-methoxy-estradiol-17 beta).

Radioactively labeled steroids undergo decomposition processes, which are dependent on time, storage conditions (temperature, solvents etc.), degree of labeling etc. This communication shows that several decomposition products of 17 alpha-ethynyl-11 beta-methoxy-estradiol-17 beta (R 2858) bind to rat uterine cytosol in a way that would be interpreted as 'specific receptor binding' if some of these compounds were present in the ligand solution used for estrogen receptor determination. Thus, the binding was charcoal-resistant and displaceable with an excess of unlabeled R 2858, and the percentage of binding was of significant magnitude to seriously interfere with receptor measurements.