The CD40 cell surface antigen and member of the tumor necrosis factor (TNF) receptor superfamily is expressed in many cell types, including normal and neoplastic B cells. Signaling through CD40 induces B cell proliferation, differentiation and, in some circumstances, protects the B cell from apoptosis. Lymphoblastoid cells (LCLs) resemble the malignant B cells that comprise the Epstein-Barr virus (EBV)-associated posttransplant lymphoproliferative disorders, in that the cells bear a highly activated phenotype and, unlike most other EBV positive tumor cells, express the majority of latent EBV genes. In this study, we use assays of cell viability, proliferation, cell cycle and apoptosis to demonstrate that ligation of the CD40 receptor in EBV-transformed LCLs inhibits their growth. The process does not involve apoptosis, but is characterized by reduced S-phase entry from G0/G1. A better understanding of the negative effects of CD40 ligation in these cells may offer clues for the development of novel therapies in EBV-related B cell disorders.