The C-terminal regions of the human and the murine c-MYC consist of a common conserved sequence, the amino acids (a.a.) 418-439 with one terminal exchange (C438G). The pre-C-terminal region of both proteins, a.a. 408-417, exhibit four exchanges. A commercially available monoclonal antibody, 9E10, raised against the C-terminal a. a. 408-439 of human c-MYC, is declared to recognize specifically and exclusively human c-MYC. However, in an immunofluorescence assay we observed, in addition to the reaction with a human cell line (SV80), reactivity with the murine cell line L929. In analogy, a rabbit polyclonal antiserum raised against a peptide which corresponds to the murine pre-C-terminus of c-MYC, a.a. 408-417, showed also cross-reactivity in immunofluorescence. The immunostaining with both anti-bodies in the human and the murine cell line was competed by the peptide, corresponding to the murine pre-C-terminal a.a. 408-417, whereas the staining of both cell lines with an antiserum raised against the conserved N-terminal region of c-MYC was not competed by this peptide. The cross-reactivity of 9E10 with murine c-MYC was confirmed by Western blot using two additional cell lines. In conclusion, our findings indicate that 9E10 which is generally regarded as specific for human c-MYC cross-reacts with denaturated murine c-MYC.