Mouse myeloid leukemic M1 cells are induced to differentiate by various differentiation inducers. Activity for inhibition of induction of differentiation of M1 cells (l-activity) has been found in conditioned medium of variant M1 cell clones resistant to differentiation inducers, and this l-activity has been shown to be closely associated with resistance of the cells to differentiation inducers. In this work, the l-activity in the conditioned medium of the resistant M1 cells was shown to bind to Carboxymethyl-Sepharose CL-6B and to be eluted with 0.27–0.4 m NaCl. The profile of gel filtration of l-activity from Sephadex G-200 indicated considerable heterogeneity in molecules with l-activity; the apparent molecular range of the main l-activity was 60,000–80,000. On chromatofocusing, the l-activity was eluted with Polybuffer 96-acetic acid at pH 8.8–9.0. The l-activity was inactivated by treatment with trypsin or by heating at 75°C for 30 min. Therefore, the main l-activity seemed to be due to a basic protein(s).

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This work was supported by a Grant-in-Aid for Cancer Research from the Ministry of Education, Science, and Culture of Japan. This paper is a part of the doctoral thesis of Junko Okabe-Kado at Tohoku University, Japan.

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