The synthesis and biological evaluation of the Forssman antigen pentasaccharide and derivatives thereof by using a one-pot glycosylation and polymer-assisted deprotection is described. The Forssman antigen pentasaccharide, composed of GalNAcα(1,3)GalNAcβ(1,3)Galα(1,4)Galβ(1,4)Glc, was recently identified as a ligand of the lectin SLL-2 isolated from an octocoral Sinularia lochmodes. The chemo- and α-selective glycosylation of a thiogalactoside with a hemiacetal donor by using a mixture of Tf2O, TTBP and Ph2SO, followed by activation of the remaining thioglycoside, provided the trisaccharide at the reducing end in a one-pot procedure. The pentasaccharide was prepared by the α-selective glycosylation of the N-Troc-protected (Troc=2,2,2-trichloroethoxycarbonyl) thioglycoside with a 2-azide-1-hydroxyl glycosyl donor, followed by glycosidation of the resulting disaccharide at the C3 hydroxyl group of the trisaccharide acceptor in a one-pot process. We next applied the one-pot glycosylation method to the synthesis of pentasaccharides in which the galactosamine units were partially and fully replaced by galactose units. Among the three possible pentasaccharides, Galα(1,3)GalNAc and Galα(1,3)Gal derivatives were successfully prepared by the established method. An assay of the binding of the synthetic oligosaccharides to a fluorescent-labeled SLL-2 revealed that the NHAc substituents and the length of the oligosaccharide chain were both important for the binding of the oligosaccharide to SLL-2. The inhibition effect of the oligosaccharide relative to the morphological changes of Symbiodinium by SLL-2, was comparable to their binding affinity to SLL-2. In addition, we fortuitously found that the synthetic Forssman antigen pentasaccharide directly promotes a morphological change in Symbiodinium. These results strongly indicate that the Forssman antigen also functions as a chemical mediator of Symbiodinium.