Soluble membrane fractions derived from Raji cells trigger lymphocytes of Epstein-Barr virus (EBV)-seropositive, but not EBV-seronegative, individuals to release a lymphokine that inhibits leukocyte migration. The reaction can be blocked by the sera of patients with EBV-DNA-carrying tumors, Burkitt lymphoma, or nasopharyngeal carcinoma. Absorption of these sera with EBV-positive, but not EBV-negative, cells abrogates their blocking activity. These findings suggest that the antigen responsible for the leukocyte migration inhibition reaction is an EBV-encoded or an EBV-induced membrane component. The antigen is not identical with EBV-associated nuclear antigen or any other known antibody-detected EBV antigen.