Human retroposons of the Alu family have an internal promoter for RNA polymerase III also present in tRNA genes, but Alu are poorly transcribed by this polymerase in somatic cells in vivo, which is probably due to an efficient system of repression of the Alu transcription. The key control promoter element of the tRNA genes, known as B-box, binds basal transcription factor TFIIIC2, which initiates assembly of the full transcription complex. Previously we identified several human nuclear factors which bind to Alu Subregion covering B-box and the adjacent sequences, which can be involved in repression of Alu transcription. In this study we identified one factor, F1, as HMF1/2 and another factor, F2, as TFIIIC2-like protein reacting with B-box. Both the factors are not alu-specific, whereas the third identified Alu-binding factor, F3, seems to be specific for the Alu-specific subsequence located just downstream from B-box and can discriminate evolutionary "young" and "old" Alu subfamilies producing different numbers of transcripts in vivo. Although HMG1/2 binds to Alu in a sequence-unspecific manner, the proteins are capable of stabilizing the sequence-specific complex F3 which can be functionally significant. We believe that the factors binding to Alu B-box subregion interfere somehow with the functions of the basal factor TFIIIC2 and, hence, can be co-repressors of Alu transcription by RNA polymerase III.