The Δ-3-ketosteroid isomerase (EC 220.127.116.11) of Pseudomonas testosterone catalyzes the conversion of a variety of unconiugated Δ 5(6)- and Δ -3-ketosteroids into the corresponding Δ-3-ketosteroids. Typical examples of the reaction are the conversion of Δ-androstene-3,17-dione and Δ-pregnene-3,20-dione to the respective Δ -3-ketosteroids. Considerable information is available on the molecular properties of this enzyme, on the catalytic mechanism, and on the stereochemistry of the enzymic reaction. Based on the proposed molecular mechanism of this reaction, a series of acetylenic 5,10-secosteroids has been prepared in the belief that they might serve as substrates for Δ 5-3-ketosteroid isomerase. Abstraction of the proton at C-4 by the enzyme should then generate, via an enolic intermediate, the corresponding highly reactive conjugated allenic ketones, which might be expected to react covalently with a nucleophilie amino acid residue at the active site. This chapter is based on expected conformational similarities between the acetylenie 5,10-secosteroids and the normal Δ-3-ketosteroid substrates for the enzyme.