Epstein-Barr virus (EBV) nuclear antigen 3C (EBNA3C) is essential for efficient conversion of primary human B lymphocytes to lymphoblastoid cell lines (LCLs) and for continued LCL growth. We used a transcomplementation assay in the context of LCLs transformed by an EBV with a conditional EBNA3C to identify the EBNA3C amino acids (aa) necessary for maintaining LCL growth. Surprisingly, we found that most EBNA3C aa were essential for continued LCL growth. Only EBNA3C mutants deleted for residues within aa 507–515, 516–620, 637–675, or 676–727 maintained full LCL growth, and EBNA3C mutants deleted for residues within aa 728–732 or 910–992 maintained slow LCL growth. In contrast, EBNA3C lacking aa 180–231, which mediate RBP-Jκ association and are necessary for EBNA3C abrogation of EBNA2-induced transcription through RBP-Jκ, could not support LCL growth. Furthermore, 2 EBNA3C alanine substitution mutants within aa 180–231, which were wild-type (wt) in abrogating EBNA2-mediated transcription through RBP-Jκ, maintained LCL growth, and 2 alanine substitution mutants within aa 180–231, which were null in abrogating EBNA2-mediated transcription through RBP-Jκ, did not maintain LCL growth. This indicates that EBNA3C regulation of transcription through RBP-Jκ is critical to maintaining LCL growth. Several other EBNA3C functions also are critical for LCL growth, because EBNA3C mutants deleted for residues within aa 130–159, 251–506, or 733–909 were wt in abrogating transcription through RBP-Jκ and expression level, but did not maintain LCL growth.