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PTH-Decahydroamide (Ro 31 -9373/000) was tested for ready biodegradability according to the Closed Bottle test procedure (OECD 301D) at a concentration of 2.056 mg/L, in parallel with a 'soft' reference substance (sodium acetate) and a more challenging reference substance: "Dobanic Acid 103" (= linear alkyl benzene sulphonate). Slight (6%) apparent degradation was recorded on Day 28 and further confirmatory measurements were made at Day 35, however no further biodegradation occurred during the final week of incubation. Both reference substances were extensively biodegraded within 28 days, confirming the activity of the inoculum, and toxicity controls containing mixtures of PTH-Decahydroamide and each of the reference substances showed no evidence of microbial toxicity that might suggest that auto-inhibition had contributed to the failure of the test substance to degrade. Based on the outcome of this study, PTH-Decahydroamide is not readily biodegradable.