The fine structure of the peripheral nerve lesions, especially cellular lesions, from the field cases of Marek's disease, has been described. The morphological classification of the peripheral nerve lesions used in the previous report has been adopted. The TI-type lesion consisted of mainly uniform small lymphoid cells. The Tn-type lesion consisted of pleomorphic cells which included small, medium and large (lymphoblastic and hemocytoblastic) lymphoid cells. The lymphoid cells (mainly large

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... hoid cells) often had the nuclear pockets in the nucleus and mitoses were frequently found. The more the lymphoid cell was immature, the ITlore it increased free ribosoITles in the cytoslasITl. The heITlocytoblastic lYITlphoid cell was characterized by unusually large nucleus and numerous polysomes in the cytoplasm. The R-type lesion consisted of infiltration of small lymphocytes and plasma cells and was accompanied by fewer reticulum cells. Furthermore, fibroblasts, mast cells and macrophages were occasionally found in these lesions. Macrophages were active and phagocytized various kinds of materials, such as myelin debris, lipids and cellular debris. Cytoplasmic bridge between macrophage and plasma cell was occasionally found in the R-type lesion.