Disclosed are a highly efficient method for production of heterologous proteins performed by utilizing microorganisms, as well as fusion proteins, and an antiserum. The method includes a method for production of a protein (A) in the form of a fusion protein, comprising the steps of (a) preparing a DNA which codes for a fusion protein comprising the peptide chain forming the protein (A) and the C-terminal peptide or its fragment (B) of the Cry proteins produced by Bacillus thuringiensis, and (b) introducing the DNA into a host bacterium to transform the same, and (c) allowing the fusion protein to be expressed in the transformed host bacterium, as well as a method for production of the protein (A) itself comprising a further step of removing the peptide chain (B) from the fusion protein obtained.