Test strips are provided for detecting even small amounts of blood or other peroxidatively active substances in body fluids; the strips comprising a carrier containing a hydroperoxide, a chromogen, and as an activator, a compound of the formula WHEREIN R1 is a hydrogen atom or a methyl radical and benzene and/or pyridine rings are fused on at least one of the positions indicated with c, (d,e), f and g, provided that two adjacent rings must not simultaneously each contain a cyclic nitrogen atom, and wherein the aromatic compounds (1), apart from the positions indicated by H and R1, can be substituted by lower alkyl radicals which together can also form a hydroaromatic ring.