The application discloses a gene fragment which comprises a DNA sequence coding for an amino acid sequence of a constant region of feline immunoglobulin λ chain; a gene fragment which comprises a DNA sequence coding for an amino acid sequence of a constant region of feline immunoglobulin κ chain; a gene fragment which comprises a DNA sequence coding for the constant region of feline immunoglobulin γ chain; a recombinant DNA molecule coding for an amino acid sequence of a mouse-cat chimeric antibody which comprises a gene fragment coding for an amino acid sequence of a variable region of a mouse immunoglobulin and a gene fragment coding for an amino acid sequence of a constant region of a feline immunoglobulin wherein the latter gene fragment is linked to the 3' site of the former gene fragment; a polypeptide of a mouse-cat chimeric antibody which is expressed from a cell transformed with an expression vector for cells wherein said recombinant DNA molecule coding for an amino acid sequence of the mouse-cat chimeric antibody is incorporated; a cat-mouse heterohybridoma which produces feline immunoglobulin; and a process for preparing a feline immunoglobulin gene.