A gene fragment which comprises a DNA sequence coding for an amino acid sequence of a constant region of canine im­munoglobulin lambda chain; a gene fragment which comprises a DNA sequence coding for an amino acid sequence of a constant region of canine immunoglobulin kappa chain; a gene fragment which com­prises a DNA sequence coding for the constant region of canine immunoglobulin gamma chain; a recombinant DNA molecule coding for an amino acid sequence of a mouse-dog 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 canine immunoglobulin wherein the latter gene fragment; a polypeptide of a mouse-dog chimeric antibody which is expressed from a cell transformed with an expression vector for cells wherein said recombinant DNA molecule cording for an amino acid sequence of the mouse-dog chimeric antibody is incorporated; a dog-mouse heterohybridoma which produces canine immunoglobulin; and a process for preparing a canine im­munoglobulin gene.