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Characteristics of a new candidate for the digoxin-like material (DLM).Steroid has 23 carbons.Phosphocholine ester.Cholesterol not a precursor.May function as endogenous potassium sparing hormone.We describe a novel steroid, which we have named “Ionotropin”. Its unique features are:  it has 23 carbon atoms and  it is a phosphocholine ester. There are no other known mammalian steroids with either structural feature. Ionotropin cross reacts with digoxin-specific antibodies and may be the long-sought, endogenous, mammalian digoxin-like material (DLM). Using LC–MS, we identified three other phosphocholine steroids in serum. Two of these steroids also cross-react with digoxin specific antibodies.In adrenal extracts, we found both phosphocholine esters and corresponding phospho-ethanolamine steroid esters. There are no other known phosphoethanolamine steroid esters. Together, these 8 compounds define a biosynthetic pathway from 7-dehydropregnenolone to Ionotropin. Ionotropin may be the only steroid hormone not synthesized with cholesterol as a precursor. Finally, we propose that Ionotropin serves as the endogenous potassium sparing hormone. Ionotropin provides a new understanding of renal, cardiac, gonadal and placental function.