Recent studies demonstrated that HLA-G transcription is in some cells silenced by epigenetic mechanisms as DNA methylation and histone modification. Accordingly HLA-G gene transcriptions can be activated in such cells by demethylating agent or by inhibitors of histone deacetylation. In addition to epigenetic alterations HLA-G gene transcription can be activated by stress. In the present study these aspects of HLA-G expression are re-examined and a new inhibitor of histone deacetylation (valproic acid) and hypoxia mimetic chemical (CoCl2) are included. The highest activation of HLA-G transcription was achieved by treatment of choriocarcinoma JAR and lymphoblastoid RAJI cell lines with demethylating agent 5-aza-2 - deoxycytidine. Treatment of JAR and RAJI cells with histone deacetylase inhibitors (sodium butyrate and valproic acid) also enhanced HLA-G transcription. Nevertheless this increase in HLA-G expression was low as compared with activation by 5-aza-2 - deoxycytidine. The hypoxia mimetic agents (desferrioxamine or CoCl2) had no detectable effect on HLA-G gene transcription in examined cells. Relatively high increase of HLA-G transcription was detected in JAR and RAJI cells exposed to heat shock treatment. Interestingly heat shock induced high expression of HLA-G6 transcript in JAR cells. Heat shock treatment had no effect on alternative splicing of constitutively expressed HLA-G mRNA in choriocarcinoma cell line JEG-3. HLA-G1 protein expression was induced in JAR and RAJI cell lines by 5-aza-2 - deoxycytidine. In agreement with the differences in the levels of HLA-G transcripts JAR cells express more of HLA-G1 protein than RAJI cells.