Three brothers from one family died of Hodgkin disease and a fourth brother from a diffuse malignant non-Hodgkin lymphoma. This patient exhibited a constant deficiency of serum immunoglobulins and elevated antibody titers to Epstein-Barr viral antigens. Epstein-Barr virus DNA sequences were detected in DNA isolated from lymph node biopsies from two of the patients. Initially, no abnormalities in the numbers of B and T cells could be detected. Peripheral blood lymphocytes of the patients did not react in the mixed lymphocyte culture assay. We suggest that an immune deficiency to Epstein-Barr virus may favor the proliferation of malignant lymphocytes after Epstein-Barr viral infection. Monoclonal lymphoid B cell lines established spontaneously in vitro from a lymph node biopsy specimen and from peripheral blood specimens from two of the patients. The cells harbor Epstein-Barr viral DNA sequences in multiple genome equivalents and express Epstein-Barr nuclear antigen. The cells contain a tenfold increased level of c-fgr-related RNA transcripts compared with peripheral blood lymphocytes of healthy adults. No obvious amplifications or translocations of the c-myc, c-abl, or c-fgr gene could be detected.