The novel bromoacetyl ecdysteroid IV, (20R,22R)‐2β,3β,14α,20,22,25ξ‐hexahydroxy‐26‐(3‐bromoacetoxypropyl)‐5β‐cholest‐7‐en‐6‐one, BAEIV, has been synthesized by extending the side chain on C26 of 20‐OH‐ecdysone. BAEIV meets all the requirements for an affinity‐labelling reagent. It reacts with the partially purified ecdysteroid receptors of Drosophila melanogaster rapidly and almost quantitatively. Reactions require only micromolar concentrations of BAEIV. The rate of the affinity‐labelling reaction is determined by the association of BAEIV with the ecdysteroid receptor. The value of the apparent reaction rate constant is very similar to that of the association rate constant for the binding of 20‐OH‐ecdysone to the ecdysteroid receptor. Product analysis of the reaction of [14C]BAEIV with the ecdysteroid receptor revealed two labelled peptides having molecular masses 150 kDa and 90 kDa. The smaller peptide is possibly a proteolytic fragment of the larger peptide. The identification of a 150‐kDa peptide by chemical affinity labelling of the ecdysteroid receptor agrees with previously reported photoaffinity‐labelling results from our laboratory. The results also demonstrate that the ecdysteroid receptor of D. melanogaster has a molecular mass higher than all other vertebrate steroid hormone receptors studied so far.