Cell-free extracts of the protozoon Astasia ocellata show laminaribiose phosphorylase activity. The enzyme catalyzes the reversible phosphorolysis of laminaribiose (and higher laminarisaccharides) to glucose and a-d-glucose 1-phosphate. At equilibrium, synthesis of laminaribiose is favored. For synthesis, a-d-glucose 1-phosphate is the only known glucosyl donor. Possible acceptor substrates are ß-d-glucopyranose and di- or trisaccharides (or derivatives) which contain a nonreducing ß-d-glucopyranosyl residue, the transferred glucose residue being attached by a (1 ? 3)-linkage. In the presence of arsenate, laminaribiose is completely converted into glucose. © 1967.