CD69 is an early marker of lymphoid cell activation. The authors report on an up-regulation of CD69 in splenic B and T cells of C57Bl/6 mice after administration of lipopolysaccharide (LPS) or microbial immunosuppressive/mitogenic (ISM) proteins produced by C. albicans (p43) and African Swine Fever Virus (p36). This up-regulation of CD69 was observed 6 and 24 h after mitogenic treatments. The same pattern of increased CD69 expression was observed in the lymph nodes of mice treated with p43 or LPS, whereas p36 treatment failed to induce increased CD69 expression in this organ. Intracellular calcium mobilization was induced in splenic B and T lymphocytes after incubation of total spleen cells with LPS, p43 or p36. This increase was higher in B than in T cells. Increased calcium mobilization was also seen in lymph node B cells after incubation with p43 or p36 and in lymph node T cells after p43 stimulation. Up-regulation of CD69 expression on B and T cells was also observed after in vitro stimulation of spleen cells with the three mitogens used. Similar results were obtained with culture supernatants of macrophage/monocyte (M phi) cells activated with LPS (LPS/M phi CS). Stimulation of M phi cells with LPS or with the ISM proteins is demonstrated by the increased production of nitrites by these cells. The increased in vitro expression of CD69 was, however, not abolished by monoclonal antibodies to M phi cytokines such as IL-6, IL-10 or TNF alpha. No increased expression of CD69 was found in vitro on purified B or T cells, even when mixed upon stimulation with p43, p36, LPS or with LPS/M phi CS. However, an increase in the expression of CD69 was observed on B cells co-cultured with M phi cells after treatment with LPS or p36. All three mitogens failed to induce increased CD69 expression on cultured T cells mixed with M phi cells.