AbstractAn ecdysteroid‐binding protein has been demonstrated in nuclear and cytosolic extracts of cultured Drosophila cells (Kccells). Attempts to purify the binding moiety have been hampered because of the small number of binding sites (ca 1,000/cell) and sensitivity of the ecdysteroid binding moiety toward salt (dissociation at 150 mM). Recently 3α[3H]kaladasterone and 3α[3H]muristerone A have been synthesized. The binding kinetics of these two ecdysteroids are similar to ponasterone A. Photoaffinity labeling of the ecdysteroid receptor in Kc cells was attempted using the 3α[3H]kaladasterone, and standard protein chromatographic techniques have been used to examine the 3α[3H]muristerone A‐receptor complex, which is less sensitive to salt dissociation. Attempts to characterize the protein to which these ligands have been attached will be di