Concanamycins, isolated from the mycelium of Streptomyces diastatochromogenes S-45, inhibit the proliferation of concanavalin A-stimulated mouse splenic lymphocytes. Previous structural elucidation revealed concanamycin A (1a) is the second example of 18-membered macrolide antibiotics following borrelidin. This communication describes the structures of concanamycins B (1b) and C (1c). Concanamycin B (1b), an amorphous white powder (C₄₅H₇₃NO₁₄), was analyzed via alkaline degradation (affording anhydroaglycone P2 and sugar S1), ozonolysis, and NMR: P2 differs from 1a's anhydroaglycone P1 by a methyl group replacing the ethyl group at C(8), with the same sugar-aglycone binding mode as 1a; NMR confirmed a 6-membered hemiketal ring (C-21/C-25 hydroxyl) with trans-diaxial H-24/H-25. Concanamycin C (1c), colorless thin plates (C₄₅H₇₄O₁₃) from ethanol, was identified via alkaline degradation (yielding P1 and sugar S2, 2-deoxy-D-rhamnose) and NMR as a decarbamyl derivative of 1a with a hemiketal ring. Thus, concanamycins A, B, and C are novel 18-membered macrolide antibiotics consisting of an α,N,i,o-unsaturated lactone ring, a hemiketal-forming long side chain, and 2-deoxy-D-rhamnose.