B lymphoblastoid cell lines immortalized with P3HR1/633 Epstein-Barr virus (EBV), which has a deletion in the EBV nuclear antigen leader protein (EBNA-LP) gene, were transfected with a vector expressing wild-type EBNA-LP. The EBNA-LP transfectants grew out faster under G418 selection than control cells but expression of EBNA-LP made no significant difference to growth rate or saturation density of the resulting established cell lines. When the cells expressing EBNA-LP were allowed to grow to saturation and then diluted in fresh medium they underwent DNA synthesis more rapidly than control cultures.