Mureidomycins (MRDs) E and F were isolated from a culture filtrate of Streptomyces flavidovirens SANK 60486 which produces MRDs A approximately D. They possessed the same molecular formulae, C39H48N8O12S and very similar UV, IR and NMR spectra, but differed clearly from each other in HPLC profile. From the hydrolysates of MRDs E and F, 8-hydroxy-1,2,3,4-tetrahydro-3-isoquinoline carboxylic acid and 6-hydroxy-1,2,3,4-tetrahydro-3-isoquinoline carboxylic acid were detected, respectively, which were not detected from those of MRDs A approximately D. They showed strong anti-pseudomonal activity but less active than MRD A. MRDs E and F were synthesized from MRD A and formaldehyde through Pictet-Spengler reaction.