Histological and to some extent additional enzyme-histochemical studies were conducted into 28 cases of Marek's disease with minor, moderate, and intensive lymphoma formation. The investigations were undertaken with a view to differentiating T-lymphocytes and macrophages by differentiated activity on acid unspecific alpha naphthylacetate esterase. Lymphoma specimens were cytologically evaluated under semi-quantitative or quantitative aspects. Evidence was provided to the existence of correlations between intensity of Marek's disease lymphomas and their relative contribution to lymphocytes, lymphoblasts, and macrophages. The amount of lymphoblasts was found to go up along with advancing lymphoma proliferation, while the amounts of lymphocytes and macrophages went down. Moderately pronounced lymphomas contained 16 percent macrophages on average, whereas only about 8 percent were recordable from highly pronounced lymphomas. The high presence of macrophages in less developed lymphomas is considered to reflect unspecific defence reaction against neoplastic lymphatic cells in the phase of low tumour malignancy.