Cutaneous round cell tumors have been classified as mast cell tumor (MCT), histiocytoma (HCT), lymphosarcoma, undifferentiated round cell tumors and occasionally rhabdomyosarcoma in veterinary medicine. An adult cock (Gallus domesticus) showing a large solitary integument mass raised on dorso bilateral of cervical part, extending to intercapsular and cranial mid part of the back was referred to the birds clinic of the faculty of veterinary medicine at the university of Tehran. Microscopic examination revealed sheeted cells with large, round to oval nuclei with each one containing one or more prominent nucleoli with scant cytoplasm. The myofibrils of the neck were degenerated by aggressive tumor cells. The condition was differentiated from other round cell tumors by electron microscope, histochemical staining, as well as the application of a large panel of antibodies. Polymerase chain reaction failed to confirm the involvement of both Marek\\\'s disease virus and avian leukosis virus subgroup-J. It was concluded that the tumor cells were consistent with both B-cell lymphocytes and histiocytes that unusually covered the entire dermal layer of dorsal neck skin. This unusual cutaneous lymphoma was named as lymphoblastic histiocytoma.

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