Tow novel geometrical isomers of oxazolomycin, oxazolomycins B (2) and C(3), were isolated from the fermentation broth of an oxazolomycin- producing strain, Streptomyces albus JA3453. The configurations of the triene moieties of 2 and 3 are (4'E, 6'E, 8'E) and (4'Z, 6'E, 8'E), respectively, while that of oxazolomycin (1) is (4'Z, 6'Z, 8'E). 2 and 3 exhibited potent inhibitory activity against crown gall formation with the same MIC (0.8 μg/disk) as oxazolomycin. 2 and 3 showed no antibacterial activity against Agrobacterium tumefaciens, in contrast to oxazolomycin which has specific anti-A. tumefaciens activity. The newly established GUS expression bioassay of the plant callus extracts revealed that the extracts of Phytolacca americana callus had the most potent inhibitory activity against agrobacterial plant transformation. A triterpene glycoside phytolaccoside B was isolated from the extract as a genuine plant transformation inhibitor having neither antiagrobacterial nor phytotoxic activity. We found that the inhibitory activity of oxazolomycin oxazolomycin diacetate against plant transformation was due to its inhibitory effect against the attachment of A. tumefaciens to host plants. Furthermore, oxazolomycin diacetate was found to exhibit an inhibitory activity against the rhicadhesin-mediated agrobacterial attachment to plants, not the biosynthesis of cellulose and β-1, 2-glucan which have been reported to be involved in the agrobacterial attachment. And we found that a novel bacterial acidic polysaccharide from a soil bacterium exhibited plant transformation enhanced plant transformation due to its stimulatory effect for the attachment of A. tumefaciens to host plants.