Epstein-Barr-virus-associated posttransplant lymphoproliferative disease ranges from transient lymphadenitis to aggressive lymphoma. This study characterizes an in vitro model to study the pathogenesis of this disease with a cell culture system. Five B-cell lines derived from posttransplant lymphoproliferative disease tissue were characterized with regard to immunophenotype, karyotype, molecular genetics, cytokine production, and growth regulation. All cell lines expressed CD19, CD21, CD22, CD43, and CD77, but not CD10 antigens. Immunoglobulin light chain restriction was seen in four of five cell lines, and cytogenetic abnormalities were demonstrable in three of the five. Cells proliferating in culture contained multiple Epstein-Barr virus episomes and showed lytic viral replication. All cell lines produced tumor necrosis factor-beta and interleukin-10 without evidence of autocrine growth regulatory loops involving these cytokines. No evidence of IL-1 alpha, IL-2, IL-4, IL-5 or IL-6 production was found by reverse transcriptase polymerase chain reaction. Adding 500 U IFN-alpha/ml to the culture medium resulted in 30% inhibition of [3H]thymidine incorporation.