Penicillinase plasmids, and some related extrachromosomal elements in Staphylococcus aureus, can specify resistance to inorganic ions, including Hg2+ and Cd2+ (Richmond & John, 1964; Novick & Roth, 1968; Peyru, Wexler & Novick, 1969). There is some tentative evidence that resistance to Hg2+ ions is due to the impermeability of the cells to the ions and not to a higher concentration of free -SH groups in resistant cells (Vaczi, Fodor, Milch & Rethy, 1962), but the biochemical basis of resistance to cadmium ions is unknown. The experiments described here show that there is a markedly decreased rate of uptake of Cd2+ ions by resistant cells when compared with strains that lack the cad-r gene.