Abstract: The invention relates to new compositions which contain 0.5 to 90% by weight of a phthalazine derivative having the general formula (I), ##STR1## wherein R.sub.1 stands for hydrogen, halogen, a straight-chained or branched C.sub.1-4 alkyl group having optionally a chlorine or hydroxy substituent, a C.sub.2-4 alkenyl group, or an optionally alkyl-substituted amino, sulfhydryl or hydroxy group, andB.sub.1 and B.sub.2, which may be identical or different, have no meaning or stand for guanidine and/or hydrazine base,and/or an imidazole derivative of the general formula (II), ##STR2## wherein n is equal to zero or 4,R.sub.2 stands for hydrogen, a straight-chained or branched C.sub.1-5 alkyl group having optionally a chlorine or hydroxy substituent, or a C.sub.2-5 alkenyl group,X stands for an organic or inorganic anion, andin the compounds of the general formula (II) wherein n is equal to 4 R.sub.