A series of neutral and anionic Rh(III)-chloride compounds bearing ammonia or imidazole (Im) ligands, and isostructural to Ru(III) complexes endowed with antineoplastic activity, were synthesized and characterized spectroscopically. The X-ray crystal structure of [ImH][trans-RhCl4(Im)2] was determined. Crystal data: monoclinic, space group C2/c, Z=4, a=13.133(3), b=7.977(1), c=16.683(4) Å, β=113.84(2)°. The solution behaviour and some biological parameters of the new rhodium compounds, including cytotoxicity, in vitro interactions with DNA and in vivo antitumour activity against a tumour model, were investigated and compared with those of the corresponding ruthenium analogues