Herpesvirus-like particles in tumor cells of breast muscle of chickens with Marek's disease.

Tumors of the breast musculature of 4 chickens with Marek's disease were examined by light and electron microscopy. By light microscopy, tumors were composed of lymphoid cells which had infiltrated and separated bundles of muscle fibers. By electron microscopy, there was margination of chromatin and duplication of the inner nuclear membrane in almost all neoplastic cells of breast tumors of all 4 chickens. In addition, in one chicken the nuclei of tumor cells contained naked herpesvirus-like particles.