We have investigated the specific immune response to Epstein-Barr virus (EBV) of peripheral blood mononuclear cells (PBMC) from patients with malignant lymphoma of the thyroid. Coculture of PBMC and EBV resulted in EBV cell transformation and regression which was assayed by an EBV-induced B cell focus-regression assay technique. The EBV had been isolated from mouthwash samples. The specific immune response to EBV by outgrowth inhibition in PBMC from untreated EBV-seropositive patients with malignant lymphoma was significantly decreased when compared to PBMC from EBV-seropositive healthy subjects (p less than 0.05). This observation is at least consistent with the possibility that B-cell proliferation after continuous or recurrent EBV infection could be a causative factor or may potentiate malignant lymphoma of the thyroid.