Density dependent stimulation of thymidine incorporation in BHK2l cells by active material released from the same cells

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Abstract
Cultures of BHK2l cells continuously release into serum‐free medium, active materials which stimulate the incorporation of thymidine in non‐confluent, but not in confluent cultures of homotypic cells. The activity is not removed by centrifugation at 30,000 g for four hours, but is non‐dialysable and retained by membranes with a nominal limitation for MW 25,000. Activity is stable at 4° for several weeks, but destroyed in 30 minutes at 56°. BHK2l cells also release active material capable of enhancing the effect of added serum. This is also heat labile and its action is density‐dependent.