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Eleven transposon mutant strains affected in bile acid catabolism were each found to form yellow, muconic-like intermediates from bile acids. To characterize these unstable intermediates, media from the growth of one of these mutants with deoxycholic acid was treated with ammonia, then the crude product was methylated with diazomethane. Four compounds were subsequently isolated; spectral evidence suggested that they were methyl 12a-hydroxy-3-oxo-23,24-dinorchola-1,4-dien-22-oate, methyl 4-aza-12β-hydroxy-9(10)-secoandrosta-1,3,5-triene-9,17-dione-3-carboxylate, methyl 4-aza-9α, 12β-dihydroxy-9(10)-secoandrosta-1,3,5-trien-17-one-3-carboxylate and 4a-[3′-propionic acid]-5-amino-7β-hydroxy-7αβ-methyl-3aα,4,7,7α-tetra-hydro-1-indanone-d-lactam. It is proposed that the mutants are blocked in the utilization of such muconic-like compounds as the 3,12-dihydroxy-5,9,17-trioxo-4(5),9(10)-disecoandrosta-1(10),2-dien-4-oic acid formed from deoxycholic acid. A further mutant was examined, which converted deoxycholic acid to 12a-hydroxyandrosta-1,4-dien-3,17-dione, but accumulated yellow products from steroids which lacked a 12α-hydroxy function, such as chenodeoxycholic acid. The products from the latter acid were treated as above; spectral evidence suggested that the two compounds isolated were methyl 4-aza-7-hydroxy-9(10)-secoandrosta-1,3,5-triene-9,17-dione-3-carboxylate and 4a-[1′α-hydroxy-3′-propionic acid]-5-amino-7aβ-methyl-3aα,4,7,7α-tetrahydro-1-indanone-δ-lactam.