Rhodium(II) carboxylate (acetate, propionate, and butyrate) adducts with isonicotinic acid (Hisonic) were prepared for study. Elemental analyses and electronic spectroscopy show that the adducts contain two isonicotinic acid ligands coordinated in the axial position at the pyridinic nitrogen. The in vitro (K562 human leukemic cell line) assay and LD10 in mice results, in addition to tests of solubility, suggest that, in the presence of blood lipids or cellular membrane, the adducts dissociate into the parent compounds and the rhodium(II) carboxylate enters the cell to carry out its biological effects.