By using two colour flow cytometric analysis, we examined the proportion of B lymphocytes bearing CD5 surface antigen (CD5+B cell). The level of CD5+B cells was markedly increased in active chronic ITP and active hyperthyroid patients with Graves' disease. There was an inverse correlation between the percentage of circulating CD5+B cells and platelet counts, and a positive correlation between the level of CD5+B cells in peripheral blood and serum T3 level. These results suggested that the level of CD5+B cells might play a role in producing autoantibodies in some autoimmune diseases.