Some nuclear proteins of human HeLa and HepG2 cells are capable of binding to GCC-triplet repeats--(GCC)n > 3 in 5'-regulatory regions of a number of mammalian genes--G-C-elements. According to our previous data, nucleotide sequence (GCC)4 in promoter of mouse ribosomal protein L32 gene (rpL32) between 17 and 6 bp upstream of transcription start site interacts to nuclear proteins from HepG2 cells, and may be considered as a GCC-element. We suggest that one of those proteins, with molecular weight about 52 kDa, which may interact with rpL32 GCC-element, is a known conservative mammalian transcription factor ZF5. DNA-binding domain of ZF5 contains a few Kruppel-like Zn-fingers (Cys2His2-type) interacting with the GC-rich nucleotide sequences in 5'-regulatory regions of a number of mammalian genes. Our results (obtained by EMSA) showed that recombinant GST-ZF5 fused protein containing ZF5 DNA-binding domain specifically binds a few GS-rich sequences: (GCC)g-9riplet repeats, 5'-GCGCGC-3' (known ZF5 consensus binding site) and (more preferable) the fragment (-24...+1 bp) of rpL32 promoter. The high affinity of ZF5 DNA-domain binding with the latter may be explained by the presence in this fragment of two overlapped subsequences, each being capable of binding to ZF5: (GCC)4 and 5'-GCGCGC- 3'. Zf5 cDNA was cloned from HepG2 cells by RT-PCR method, and then used for construction of the gene expression vector. It has been shown that Zf5 cDNA expression vector specifically down-regulates (in luciferase assays) the activity of rpL32 promoter (-155...+159) including the above mentioned GC-rich subsequences by cotransfection of HepG2 cells. Therefore, our results enable us to consider GCC-elements as a novel class of ZF5 targets in 5'-regulatory regions of mammalian genes.