The antibiotic responsible for the high antiyeast and antifungal activity of a streptomycete fermentation is shown to be the 9-methyl derivative (1) of streptimidone (2). The conjugated diene system in (1) is assigned the 6E,8Z stereochemistry. Evidence is presented for the biogenetic origin of the 9-methyl group in this streptimidone homologue, and its antifungal activity is compared with that of streptimidone and other fungicides.

Critical re-evaluation of the published data, together with additional experimental evidence, establishes that protomycin, previously suggested to have the structure (3), is identical with the glutarimide antibiotic streptimidone (1).

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