PURPOSE: To accurately determine the contamination degree of a separation and purification apparatus by carrying out the transformation with a gene devoid of one of two or more kinds of drug-resistance markers and comparing with a transformant having two or more kinds of the markers.

CONSTITUTION: A specimen containing a gene A having two or more kinds of drug-resistance markers is treated with a gene separation and purification apparatus to effect the separation and purification of the gene A and, thereafter, the degree of contamination of the apparatus with the gene A is determined by the following procedures. A specimen having a gene B devoid of one or more kinds of the above two or more kinds of drug-resistance markers is treated with the above apparatus to separate the gene B. A host free from any of the drug marker is transformed with a fraction containing the gene B and the drug resistance of the obtained transformant originated from the drug- resistance marker of the gene A is compared with that of the gene B. The degree of contamination with the gene A is calculated from the comparison result.