Beta2-Microglobulin (beta2M) is the light chain of the class I HLA molecule. The serum level of beta2M is elevated in various diseases including lymphoma, inflammation, viral infections and chronic renal dysfunction. The present study addressed the possible influence of beta2M on T lymphocyte activation in vitro. Peripheral blood mononuclear cells from a group of 17 healthy subjects were examined. Stimulation with OKT3 and fibronectin in combination with 30 mg beta2M/dl resulted in a two-fold increase of cell proliferation. A similar effect was observed when OKT3 and collagen I were applied as well as when OKT3 and collagen IV were used as costimulation to T cells. The CD69 expression, measured by flow cytometry was significantly enhanced above the control level (1.52 ? 1.03% vs 33.21 ? 20.26%, p<<0.01, control group and 30 mg beta2M/dl, respectively). Together, these observations suggest that beta2M may play a role in modulating lymphocyte proliferation, possibly through modification of the CD69 molecule.