PROBLEM TO BE SOLVED: To obtain a medicine composition, a diagnosis composition and a study composition by bringing a specific cationic compound into contact with a biological sample, stabilizing a nucleic acid from the biological sample and isolating the nucleic acid. SOLUTION: A base bonded to a strong and/or weak inorganic and/or organic acid is used as an anion. A cationic compound in which a substance comprising a cationic compound and an anion is totally neutral and which is one or more cationic compounds of the formula [X is N or P; k is 1-24; Bk is a (substitute) aliphatic alkanediyl crosslinking, substituted phenyl, naphthyl or biphenyl crosslinking; R1, R2 and RBk are H, a (substitute) 1-6C alkyl, phenyl or the like; RA, RBk and RC are each H, a (substitute) 1-21C alkyl, a 1-21C alkenyl or the like] such as ethanediyl-1,2-bis(dimethyldecylammonium bromide) is brought into contact with a biological sample such as clinical sample, for example, blood, etc., to stabilize and/or isolate a nucleic acid from the biological sample.