The investigations reported here are about a carrageenan extracted from the abundant red alga, Solieria chordalis (Gigartinales) settled along the coasts of Brittany. Its structural features were characterized by GC―MS, 13C NMR and FTIR spectroscopies. The structural components of this polysaccharide are mainly a (DA2S-G4S)-type structure in association with methylated-L-carrageenan, pyruvated α-carrageenan and the minor precursor, v-carrageenen, in small amounts. The relative molecular weight of the native polysaccharide was estimated by LP-GPC as 913 kDa. The low molecular weight fractions (below 20 kDa) obtained by free-radical depolymerization and mild-acid hydrolysis presented substitution patterns similar to those of the native polysaccharide. These fractions proved to be devoid of direct cytotoxicity on Daudi (human Burkitt's lymphoma), Jurkat (human leukaemic T-cell lymphoblast) and K562 (human chronic myelogenous leukaemia) cells lines. On the other hand, they showed great immunostimulating properties: enhancement of neutrophil phagocytosis, cytotoxicity by natural killer cells, antibody-dependent cell cytotoxicity and stimulation of lymphocyte proliferation. Further to these investigations, it could be worth using the low molecular weight fractions of carrageenan from the red alga, S. chordalis, in immunotherapeutic approaches to cancer treatment.