Cell wall polysaccharides of the red algae Laurencia microcladia were isolated by alkaline treatment (KOH 1M, 10 mg NaBH4, room temperature), yielding two sub-fractions by neutralization (AcOH, pH=5.2) and EtOH precipitation, respectively A and B. Paper electrophoresis revealed the existence of only one polysaccharide in each fraction. FT-IR spectra of both polysaccharides showed signals for COH (OH: 3400-3300 cm-, CO: 1260-1000 cm-) and CHC (1300 – 1000 cm-) typical of cell wall algal polymers. An N-glyco derivative (1640 – 1560 cm-) and a low acidic content (0.34 mol %) were structural features found, while proteins were not detected. Antimitotic activity was observed on Lytechinus variegatus (green bur) larvae, with chronic values of 38.18 mg/mL and 267.99 mg/mL for polysaccharides A and B, respectively. No bradykinin or acetylcholine antagonistic effect was observed when polysaccharides were assayed in vitro in guinea-pig ileum. Preliminary results pointed to neurotoxic effect of these macromolecules (5mg/100 mL) since asymmetry of the neural tube in chick embryo was also found.