These interactions have also been suggested based on preliminary studies using NG-108-15 cell membranes, which showed that the rate of dissociation of the 5-HT3 receptor antagonist [ 3 H]RS-42358 depends upon the ligand used to prevent rebinding. 2 To characterize further putative allosteric interactions at 5-HT3 receptors, steady-state and kinetic radioligand binding studies have been conducted using the receptor agonist 1-(mchlorophenyl)biguanide. This suggests that the binding of this agonists to unoccupied sites on the receptor can increase the receptor's affinity for prebound ligands, and thereby slow their dissociation. 3,4 In this paper, we describe an X-ray structure determination of 1-(m-chlorophenyl)biguanide hydrochloride. The crystals were prepared from a commercially available compound.X-ray data for crystals of the title compound were collected by graphite-monochromatized Mo Kα radiation at 293 K. No absorption correction was applied. The structure was solved by direct methods and refined by full-matrix least-squares with anisotropic temperature factors for the non-hydrogen atoms. In both molecules, the C4, C5 atoms of the phenyl rings show relatively high temperature factors, which may be due to a localized molecular disorder. The hydrogen atoms bonded to C C8H11Cl2 N5 is monoclinic, P21/n. The unit-cell dimensions at 293 K are a = 9.3569(9), b = 13.6832(14), c = 18.4879(19)Å, β = 98.411(2)˚, V = 2341.6(4)Å 3 , Dx = 1.408 g/cm 3 , and Z = 8. The R value is 0.043 for 4129 reflections. The monoclinic unit cell contains two crystallographic independent molecules, A and B. The Cl(1) and Cl(4) ions are coordinated to 5 and 6 N atoms, respectively. In the crystal, there are two hydrogen bonds between N5A and N3B:N5B and N3A of the neighboring molecules, and constitute the major intermolecular attraction and packing force. These two molecules make a centro-symmetric dimer.