Plasma levels of the soluble fragments of Fc epsilon RII/CD23 (sCD23/IgE-binding factor) were measured to assess the level of activation of B lymphocytes associated with Epstein-Barr virus infection in 28 patients who received living-related liver transplantation and were treated with FK506 and steroids. In 6 patients with symptoms of EBV infection (EBV-related disorders), the plasma concentration of sCD23 increased to more than 9.8 ng/ml at the onset of symptoms. In a patient with B cell lymphoma, the plasma levels of sCD23 increased significantly when peripheral lymphadenopathy was noticed, and remained more than 10 ng/ml during the terminal period. In 4 of 6 patients, the increase of plasma levels of sCD23 preceded the increase of anti-EBV capsid antigen IgM. In the other 2 of 6 patients, there was no significant increase of the antibody, despite the integration of EBV DNA in the mononuclear cells in their ascites. The plasma levels of sCD23 of the patients without symptoms of EBV infection did not exceed 7.5 ng/ml. In contrast, the proportion of CD20+/CD23+ B lymphocytes in peripheral blood mononuclear cells was not significantly different in the patients with EBV-related disorders and those with latent asymptomatic EBV infection. Therefore, the plasma level of sCD23 is a sensitive and useful marker of EBV-related polyclonal and/or monoclonal B cell proliferation in transplanted patients with immunosuppression.