Schwann cells and endoneurial fibroblast-like cells were quantitated for 30 weeks in both nonregenerating and freely regenerating, transected rat sciatic nerve. Immunocytochemical recognition of S-100 protein was used as a marker for Schwann cells and other immunocytochemical and histological methods in the differentiation of S-100 protein-negative endoneurial cells in cross sections of the distal stump 10 mm distal to the site of transection. A marked increase in the total number of cells was observed during the first 4 weeks after the injury in both operative groups. The quantitative relationships between cell populations remained essentially the same as in normal nerves, although the proliferation of the S-100 protein-negative cell population was proportionately slightly stronger when compared to the number of these cells in normal nerves. After the initial proliferation, a gradual decrease occurred in the total number of cells per cross section. This was most marked in the non-regenerating nerves, whereas in the regenerating nerves the decrease in cell number ceased at 16 weeks. The number of Schwann cells was 3.5 times as high as in the control nerves in this phase. The method used in the present study is less laborious than morphometry employing electron microscopy. Furthermore, electron microscopic characteristics of endoneurial cells are not always reliable after nerve trauma, because normal anatomical relationships have become disturbed. This study demonstrates that S-100 protein immunocytochemistry is useful in the study of traumatic lesions of peripheral nerve.