Introduction: Several lymphoblastoid cell lines with various defects of HLA class II expression had been described, shedding light on regulation of class II gene expression. Here, we present a new lymphoblastoid cell line (LCL) with such a defect, derived from a class II-positive patient.Material and Methods: Lymphoblastoid cell line (LCL) HAJ was derived from peripheral blood lymphocytes of female patient with renal insufficiency. HLA class II expression was tested by: (a) flow cytofluorimetry using a panel of monoclonal antibodies; (b) RT-PCR using HLA-DQA1 and -DPA1 locus-specific primers. The presence of HLA-DRB1, -DQA1 and -DPA1 genes was also looked for using amplification from genomic DNA with locus-specific primers. Another LCL, PAJ, expressing cell surface HLA class II antigens, served as positive control.Results: Flow cytofluorimetry revealed presence of HLA class I antigens on both HAJ and PAJ cells. In contrast, class II antigens were absent from the cell surface of HAJ cells while they were abundant on PAJ cells. Permeabilization and fixation of cells with acetone/formaldehyde solution revealed intracellular Ki-67 antigen but no class II molecules. The genes HLA-DRB1, -DQA1 and -DPA1 were present in the HAJ genome. In RT-PCR, transcripts of DQA1 and DPA1 genes were easily detectable in PAJ but not in HAJ cells.Conclusion: Results suggest a defect of transcription of genes for all class II antigens in HAJ cells. Fusion with cells with already described defects of transcription factors should revealed the nature of regulatory defect in HAJ cells.