Transformation of normal resting B cells by the Epstein-Barr virus (EBV) leads to the establishment of permanent lymphoblastoid cell lines (LCL) which express high levels of HLA antigens and which are highly efficient in antigen presentation. Certain features of the LCL phenotype can be reproduced by transfecting EBV-negative B lymphoma (BL) cell lines with individual EBV latent genes under heterologous promoters. In this work we have analyzed a series of subclones derived from the EBV-negative cell line Louckes, stably transfected with constructs encoding EBV latent genes for their expression of HLA class II molecules. Louckes parental cells and control transfectants expressed detectable levels of HLA-DR, DQ and DP antigens on the cellular surface by cytofluorometry, but these levels were significantly increased in transfectants expressing the virus-coded latent membrane protein 1 (LMP-1). Northern blotting for the individual alpha and beta chain mRNA at each of the three HLA class II loci indicated correspondingly increased levels of HLA class II transcripts in the LMP-1 transfectants. Transfectants expressing the virus-coded nuclear antigens EBNA-1, EBNA-2 or EBNA-LP showed no significant changes in these parameters. These observations indicate that up-regulation of HLA-class II molecules can be a part of the changes induced by LMP-1 in B cells.