Plasma samples from mice bearing the BCL1 leukemia were shown to express elevated levels of neutral proteinase activity when assayed with radioiodinated casein as a substrate. Increased plasma proteinase activity reached levels 3–4-fold higher than controls. The increased levels of activity did not correlate with the degree of hepatosplenomegaly or the number of tumor cells in the blood. The onset of the increase in plasma activity correlated with the onset of the leukemic phase of the disease. These findings with the murine leukemia may be analogous to recent findings of abnormal proteinase activity in plasma from patients with acute leukemi