CD69 is a very early cell activation antigen expressed on the surface of activated immune cells. It can appear within 1-2h of activation and exhibits maximal expression levels between 18 and 24h after stimulation. In this work, the expression profile of CD69 in mice splenocytes was evaluated following exposure to the biopolymers, chitosan or alginate and the immunostimulatory factors, CpG ODN 1826 or concanavalin A. We have shown that both polymers are able to upregulate expression of CD69 on B cells and CD4+ T-lymphocytes, with alginate as the least potent stimulus. Moreover, the expression of the CD69 molecule on CD8+ T-lymphocytes was observed only in splenocytes cultured with chitosan. However, activation of lymphocytes did not result in cell proliferation. On the other hand, CpG ODN proved to be a potent B cell stimulator, as evidenced by the upregulation of CD69, but had less effect on T-cells. These results, together with previous discoveries reported in scientific literature, may contribute to the clarification of the adjuvant effect, which has been attributed to chitosan and alginate formulations or to the biopolymers itself.