The Epstein-Barr virus (EBV)-encoded latent membrane protein 1 (LMP1) and Epstein-Barr virus nuclear antigen 2 (EBNA2) are expressed in EBV-immortalized human B cells. It has previously been shown that transfection of the LMP1 and EBNA2 genes into Burkitt's lymphoma cell lines results in the up-regulation of CD23, CD21, ICAM-1 and LFA-1 cell-surface proteins. In the present study, the effects of transient expression of the LMP1 and EBNA2 genes were studied in normal primary human B cells pretreated with UV-inactivated EBV particles. To identify and purify cells which express the transfected DNA we used a gene encoding a surface molecule, CD2, as a co-transfection marker. We show that transient expression of the LMP1 gene, from heterologous promoters, is sufficient to induce cellular enlargement and up-regulation of surface expression of the activation markers CD23, CD21, ICAM-1 and LFA-1 in primary B cells. Most importantly, we show that transient expression of the LMP1 gene is sufficient to induce DNA synthesis in human primary B cells. Transient EBNA2 expression enhanced the effect of transient LMP1 expression on CD21 and CD23 cell-surface expression but, under our experimental conditions, inhibited the induction of DNA synthesis by LMP1. We conclude that activation of primary B cells with inactivated EBV particles, followed by transient expression of only two viral genes, EBNA2 and LMP1, is sufficient to reconstitute some of the early events of B-cell immortalization by EBV.