Novel reagents for the detection by immunoassay of drugs in body fluids, their preparation and use are disclosed. The reagents of the present invention correspond to the formula P-[A-D]n where: D is a drug derivative suitably selective for the determination of the presence of the target drug or drug metabolite, A is an activating linker-spacer group having an N-hydroxysuccinimide or isothiocyanate derived linking moiety, P is a poly(amino acid) or polymer capable of covalently bonding with A, and n is less than 1.
