Environmental stimuli in the progression of BHK-21 cells through the cell cycle.

An L-cell line adapted to grow in protein-free medium is a source of material that stimulates the proliferation of serum-dependent BHK cells under otherwise protein-free conditions. The material produced by the L-cells is resolvable into separate DNA-synthetic and mitotic stimulatory activities. The stimulation of mitosis is dependent upon the presence of one or more heat-labile macromolecules, which can be concentrated by ultrafiltration, and at least one dialyzable component is essential for the initiation of DNA synthesis.