AbstractThe brown algae are characterised by their accumulation of polysaccharides, the more important ones being laminarin, fucoidin and alginic acid. It has been established by Bidwell et al.(1,2), that alginic acid is the most rapidly accumulated insoluble product on photosynthesis. The rapid appearance of such a large proportion of photosynthetic carbon in alginic acid suggests a possible method for the biosynthesis of radioactive alginic acid‐C‐14 (generally labelled, G). Such a synthesis is now carried out on laboratory scale and the details are given. The radioactive alginic acid‐C‐14(G) thus synthesised finds many uses, such as (1) for quantitative determination and estimation of alginic acid present in various seaweeds and (2) for ascertaining the role alginic acid plays in the metabolism of brow