The ribosomes of Halobacterium cutirubrum, a micro-organism that grows in saturated sodium chloride and probably has an internal concentration of potassium chloride near saturation, exist as 70 s particles when isolated in saturated potassium chloride containing 0·1 M -magnesium chloride at pH 7. These particles are stable in concentrations of magnesium ions as high as 0·4 M . If the potassium ion concentration is below about 4 M , or the magnesium ion concentration below 0·1 M , the ribosomes dissociate into 52 s and 31 s subunits. The requirement by the 70 s particles for potassium ions is specific; in high concentrations of ammonium or caesium ions, many of the ribosomes dissociate into subunits, while in high concentrations of sodium ions, they dissociate and aggregate in a complex way. About 53% of the RNA of the cell is accounted for by the ribosomes, which consist of 60% RNA and 40% protein. The estimated molecular weights of the ribosome and its subunits are in the ratio 3 : 2 : 1. At low concentrations of magnesium ions in the absence of potassium ions, the ribosomes lose low molecular weight RNA and up to 75% of their protein, to produce a labile 42 s component and more stable 33 s and 22 s particles, which contain about 80% RNA. It is suggested that the 42 s, 33 s and 22 s components, with molecular weights in the ratio 3 : 2 : 1, represent essentially the RNA moieties of the whole 70 s ribosome and of the 52 s and 31 s subunits respectively, together with some structural protein. Gel electrophoresis showed that the proteins which are lost are acidic. It is concluded that in these ribosomes, the stability of the ribonucleoprotein depends on potassium ions which shield negative groups, and on magnesium ions, which provide ionic cross-links between RNA and protein.