1. Conditions are described for the enzymic iodination of ribosomal subunits from rat liver. The reaction is relatively insensitive to broad changes in the concentration of KCl, allowing subunits to be studied under conditions which minimize their dimerization. 2. Mixtures of extracted ribosomal proteins were iodinated with 125I, the proteins separated by two-dimensional gel electrophoresis and the radioactivity in each protein was determined. Thus 19 out of 23 of the proteins of the small subunit and 25 out of 33 of the proteins of the large subunit were labelled. Iodination should therefore be a suitable method for studying the topography of the ribosomal proteins of rat liver. 3. When the intact 40S subunit (rather than the extracted mixture of proteins) was iodinated, 18 of the 19 proteins were still labelled. However five of these were labelled less strongly than before. When the intact 60S subunit was iodinated, 17 of the 25 proteins were still labelled, although six of these were labelled less strongly. 4. These results show that in rat liver most of the ribosomal proteins of both subunits are at least partially at the surface of the particles. They are also consistent with the idea that the proportion of the ribosomal proteins in the interior of the particle may be greater for the 60S subunit than for the 40S subunit.
