Chemical and detailed 1H and 13C n.m.r. and mass spectral studies lead to the revised structure (8){9,11-dihydroxy-22-methyl-6,23-dioxa-26-thia-2,17,27-triazatricyclo[17.5.2.14,7]tetracosa-4,7(27),12,14,19-pentaene-3,18,24-trione} for griseoviridin, a metabolite of Streptomyces griseus. The known chemistry of the antibiotic is shown to be compatible with the proposed formulation. X-Ray analysis on griseoviridin methanol solvate by direct methods confirmed these assignments, and, with the evidence of the antibiotic's hydrolysis to D-cystine, gave the absolute configuration of the molecule. The n.m.r. data suggest that griseoviridin adopts in solution one predominant conformation, probably the same as in the crystal lattice. The relationship of griseoviridin to other related cyclic microbial peptides and possible biogenetic implications are discussed.