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Abstract Medium harvested from cultures of mouse L‐cells contains “conditioning factor activity” (CFA) that may be detected by its ability to stimulate colony formation by mouse marrow cells in culture. The active material has been characterized and appears to be a glycoprotein with properties similar to those reported for the colony‐stimulating activity in human urine. Characterization of trypsin‐digested material indicated that only part of the molecule is essential for biological activity. The CFA has been partially purified from serum‐free L‐cell conditioned medium, and evidence has been obtained that radioactivity derived from labelled valine or glucosamine may be incorporated into the factor. L‐cell conditioned medium appears to be a convenient source of partially‐purified, highly active material, suitable for use in studies on its mechanism of action.
Austin et al. (1971) studied this question.