The structures of the major members of a new family of important thiopeptide antibiotics, A10255B (l), A10255G (la), and A102555 (16), produced by Streptomyces gardneri (NRRL 15537), are described. Selective chemical degradation in combination with NMR, FABMS, and CID methods on the degradation producta was required to solve these structures. Methanolysis of 1 resulted in the isolation of 4-carbomethoxy-2-propionyloxazole (8) and dimethyl sulfomycinamate (9) as well as N-( (acetamidomethyl)tlyl)-l-(carbomethoxythiamlyl)ethanamide (11) after acetylation. Vigorous treatment with acid produced berninamycinic acid (10). Trifluoroacetolysis led to cleavage at the six dehydroalanine (deala) residues to give a complex and highly modified pentapeptide 12 which was sequenced by CIDMS and NMR techniques. Compound 12 was composed of the following: sulfomycinamic acid, threonine, l-(4-carboxyoxazolyl)-l-aminopentene unit (dehydronorvaline masked by oxazole at ita carboxyl group), 2-(aminomethyl)thiazole-4-carboxylic acid, and 2-(l-aminoethyl)-4-carboxamidothiazole. FABMS and base hydrolysis showed that 1 had a deala tetrapeptide side chain. Antibiotics 15 and 16 each had a masked dehydrobutyrine in place of the dehydronorvaline present in 1, and 16 had a single amidated deala as a side chain.