The distribution of ribosomes between their various functional states (native subunits, unprogrammed ribosomes, monoribosomes and polyribosomes) was analyzed in livers of cadmium-treated (CdCl2, 20 umol/kg, l h) and control mice. The ribosomes with double-labelled RNA were separated by sucrose density-gradient centrifugation in zonal rotor, and monomeric ribosome fraction was subsequently isolated and analyzed by selective dissociation of unprogrammed ribosomes. The analysis shows that the increase in monomeric ribosomes, occurring during polyribosome disaggregation in livers of cadmium-treated mice, entirely due to the increase of the unprogrammed ribosome fraction at the expense of polyribosomes. Protein synthetic activity of polyribosomes in vivo in livers of cadmium-treated and control mice were compared by double labelling of nascent and soluble polypeptides with [14C]-and [3H]-leucine. Incorporation of radioactivity was relatively higher in nascent polypeptides and lower in soluble polypeptides in cadmium-treated as compared with control mice. The results indicate that cadmium inhibits protein synthesis in livers of mice, affecting both the rate of initiation and the rate of elongation, but decreasing the former more than the latter.