In the present work, polysomal structures, estimated to contain from two to seven 60 s monomers ‡ ‡ The values of 60 s, 45 s and 35 s have been used here for the sedimentation coefficients, of the mitochondrial ribosomes and their major and minor subunits, respectively. These values were estimated on the basis of the hydrodynamic behavior of newly synthesized particles, labeled during a 2-hr [5- 3H]uridine pulse in the presence of 0.1 μg actinomycin D/ml., relative to that of the cytoplasmic ribosomal subunits. It is likely, however, that the mature mitochondrial ribosomes not bearing messenger RNA have a sedimentation coefficient somewhat lower than 60 s. A value of about 56 s was in fact estimated after mild RNase treatment (presumably apt to digest most of the attached mRNA) of [ 3H]uridine-labeled particles and of ribosomes labeled with [ 3H]leucine in their nascent protein chains. , have been identified in HeLa cell mitochondria and some of their properties have been investigated. The study of mitochondrial polysomes has been facilitated by the use of RNase inhibitors, which reduced the contamination of these polysomes by endoplasmic reticulum-bound polysomes, and by the use of appropriate antibiotics, which allowed the selective labeling of either the nascent chains or the RNA components of the mitochondrial structures. Mitochondrial polysomes have revealed an unusual resistance to degradation by RNase; furthermore, only a partial dissociation of ribosomes into subunits and very little release of nascent chains were observed in the presence of even high concentrations of EDTA. The evidence derived from enzymic tests and from the comparison with the effects of RNase and EDTA on endoplasmic reticulum-bound polysomes, has suggested that this unusual behavior of mitochondrial polysomes is due to the nature of their polypeptide products, which makes the nascent chains particularly “sticky.” These chains would thus, on one hand, interact by secondary bonds with the two ribosomal subunits, stabilizing the ribosomes against dissociation by removal of Mg 2+; on the other, they would interact with other chains on the same polysome, thus stabilizing the whole structure against degradation by RNase. The analysis of the distribution of mitochondrial rRNA among different ribosome structures has indicated that, under the present conditions of cell growth and of subcellular fractionation, about 50% of mitochondrial ribosomes are recovered in polysomes, about 20% as monomers and the rest as free subunits.