Algal polysaccharides are indigestible and exhibit unusual biochemical and fermentative characteristics from which stem interesting biological effects such as antitumoral, immunostimulating and/or prebiotic effects. In this study, we aimed to determine whether oligosaccharides obtained from alginates and laminarans also have such biological activities and can thus be considered as functional foods. The chemical structures of the oligosaccharides were determined using NMR. Both the fermentation and the effects on microbial populations of oligo-alginates and oligo-laminarans were investigated using batch incubations with, and continuous culture of, human faecal bacteria. The kinetic and intensity of fermentation were measured by continuous monitoring of gas production and determination of final pH value, respectively. Effects on intestinal flora activity and composition were determined via metabolite quantification and main bacterial genera enumeration. Cytotoxic, proliferative and differentiating effects were estimated after exposure of epithelial (Caco-2), monocytic (THP1) and lymphocytic T (Jurkat) cell lines. Despite very different biochemical structures, the two oligo-alginates exhibited similar fermentation patterns. As with native alginates, they required adaptation prior to their metabolism. However, this adaptation did not result in any change in the global bacterial composition. No noticeable biological effect was detected for oligo-alginates, In contrast to native laminarans, oligo-laminarans did not require adaptation prior to their fermentation. Propionate production was stimulated but no significant modification of the balance between the main bacterial genera was observed during continuous culture of human fecal flora. Oligo-laminarans exhibited slightly inhibitory effects on Caco-2 cells, inhibited mononuclear cell proliferation and stimulated the expression of ICAM-1 by monocytic cells. This last property appears promising, and may allow algal oligosides to be used as functional foods and/or components.