We found a new protein haponin (an HLDF-alike protein) in promyelocyte HL-60 cells that is immunoreactive to polyclonal antibodies against HLDFbeta. Determination of the partial primary structure of the protein allowed us to reveal an immunogenic peptide of haponin and, on the basis of the amino acid sequence of this peptide, the degenerate primers were synthesized, which enabled us to clone the full-size cDNA of haponin. The stable heterologous expression of this cDNA in E. coli cells (Rosetta strain) was obtained. Preparations of natural and recombinant proteins exhibited antigenic cross-reactivity to polyclonal antibodies against this peptide.