1. Acetylcholine occurs naturally in animals, flowering plants, moss, fungi and bacteria but has not been reported in marine algae, although choline is common in these organisms. During the investigation of British marine algae for compounds with pharmacological activity, an acetylcholine-like compound was detected in extracts of Laurencia obtusa (Huds.) Lamour. Identification was performed via pharmacological analysis (using isolated muscle preparations and acetylcholinesterase incubation) and gas-liquid-chromatography. Results showed the pharmacological activity was due to acetylcholine, which was destroyed by acetylcholinesterase; gas-liquid-chromatography confirmed its presence with a content of 24 nmole/g fresh weight. The acetylcholine was derived from L. obtusa as samples were free from epiphytes and contaminants. Its role in this species and occurrence in other algae are unknown, with further research needed on its biosynthesis and degradation in algae. 2. Compounds with strong UV absorption (310–360 nm) exist in marine plants and animals; previously, palythine (λmax 320 nm) was isolated from Chondrus yendoi Yamada et Mikami. Further studies on UV-absorbing compounds from this alga yielded a new compound with λmax 333–334 nm, named shinorine. The alga was extracted with 50% ethanol, and shinorine was purified via activated charcoal and Dowex 50W-X8 columns. Spectral (UV, IR, field desorption MS, 1H/13C NMR) and chemical analyses (ammonium hydroxide treatment, acid hydrolysis) revealed its molecular formula (C₁₃H₂₀N₂O₈) and structure (same skeleton as palythine, with serine). Acid hydrolysis produced glycine and serine. Shinorine is a new amino acid with an imino-cyclohexene ring, similar to other UV-absorbing compounds in Rhodophyta. 3. A preliminary study focused on the polysaccharide from alkaline extraction of Acanthophora spicifera (Vahl) Boergesen, a red alga growing with agarophytes/carrageenophytes. The alga was extracted with 2% NaOH (80–90°C), purified via isopropyl alcohol precipitation. IR spectroscopy showed the polysaccharide’s spectrum was unlike agar but similar to alkali-treated λ-carrageenan (strong 1240 cm⁻¹ sulfation band, intermediate 3,6-anhydrogalactose content, resolved peaks at 810/840 cm⁻¹ and 1025/1075 cm⁻¹). A 1% solution had a viscosity of 15.66 cps at 75°C and was non-gelling. The polysaccharide is concluded to be a λ-carrageenan structurally modified by alkali treatment.