The paper describes the immunohistochemical and ultrastructural features of normal posterior root ganglia in a group of humans aged 1 day to 80 years and compares the findings with those seen in the ganglia of normal rats of various ages, some of which underwent permanent traumatic lesions of the sciatic nerve. In humans, cells with the immunohistochemical reactions of macrophages are present in small number at birth, most of them having an endoneurial position. Subsequently, their number increases and more of them are seen around neurons, where their processes intermingle with those of satellite cells. Ultrastructural studies confirm that, in addition to interstitial cells, a small number of cells in satellite position have features of mesenchymal cells. In this respect, human sensory ganglia differ from those of rodents and this difference may explain why no nodules of Nageotte can be found either in ageing animals or after a permanent damage to the nerve has produced considerable cell loss. Other features observed in human ganglia, but absent in rats, are multiple layers of satellite cells surrounding each neuron and desmosome-like structures between satellite cell processes. Previous studies describing maturation of the satellite-nerve cell complex in animals are confirmed. In addition, the present investigation shows that, in human ganglia, satellite cells acquire a more elaborate structure than in rodents. It is also suggested that mesenchymal cells may play a role in the trophism of nerve cells and their removal after irreversible damage.