It is well known that patients with silicosis are frequently associated with hyperglobulinemia, RA or PSS. It is also suggested that silica can drive and activated the immune system. In this study, we intended to measure the percentage of CD4+CD45RA+ cells in the peripheral blood of patients with silicosis and to investigate whether silica can actually activate human lymphocytes in vitro or not. Peripheral blood from 45 patients with silicosis was stained with OKT4 and 2H4 monoclonal antibodies, and measured by FACS analysis at 1st day and also checked similarly after 6 days-incubation without patients' sera. The intracellular Ca++ level was also checked after incubation with the normal human lymphocytes with silica by FACS analysis, using Fluo3-AM. CD4+CD45RA+ T cells decreased significantly in number. One to 5 minutes after the incubation with silica, intracellular Ca++ level increased markedly, which means the activation of lymphocytes in vitro. These results revealed that silica can activate human lymphocytes in vitro, but the role in vivo remains to be clarified.