The isolation of a third crystalline antibiotic, erythromycin C, from Strehtoinyces evythreus is described. Some of the physical, chemical and biological properties of the antibiotic are discussed and a partial structure proposed. The possible competition between Curtius rearrangement of diphenylcarbamyl azide (IC) and displacement of its azide function by extraneous bases has been examined. In pyridine or ethanol as solvent the Curtius reaction was dominant, the sole products of aminolysis being l,l-diphenyl-4-substituted semicarbazides (IV). When various amines were employed as both solvents and reactants, effective competition with the facile intramolecular rearrangement was realized. With amines of b.p. 185' (and pK.'s of ca. 9-10) formed symmetrically substituted ureas such as VI. The trends operative within the various classes of reaction with pK. and temperature variations have been explored.