Leukemia is characterized by a neoplastic and malignant growth of cells deriving from the hematopoietic organ. The present investigation is an attempt to evaluate the growth activity of leukemia cells in quantitative terms for comparison with the growth of the homologous, normal hematopoietie cells. Virus-induced erythroleukemia in chicken provided the experimental material. Two different systems of measuring were chosen for assessing the growth activity. First, growth as manifested by cell

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... sion was recorded by direct counting of mitotic figures in cytological preparations. Astaldi and Mauri's (~) stathmokinetic test--i.e., the accumulation of mitotic figures at various times after colchicine treatment--was also applied. Second, the DNase activity in correlation with the cellular nucleic-acid content was deternfined. In earlier investigations we found this enzyme activity to parallel the variations in proliferative activity during regeneration of bone marrow cells (8). These findings are in agreement with other results on the DNase pattern in nonmalignant growth (3, 6) . On the other hand, a lack of parallelism between growth activity and the DNase activity has been found in several human and chemically induced experimental tumors (6, 7). One object of the present investigation was, therefore, to establish the DNase pattern during virusinduced tumor growth, a problem hitherto not attacked. MATERIALS AND METIIODS Young chickens of a White Leghorn strain (EDO) and ~-4 weeks old were used. Before and during the experiment they had free access * The following abbreviations and contractions have been used; DNA, deoxyribonucleic acid; DNAP, DNA-phosphorus; DNase, deoxyribonuclease; PNAP, pentosenucleic acid-phosphorus. t This investigation was supported in part by funds of Karolinska Institutet and the Lady Tara Memorial Trust. :