1. 1. Cytosol and extruded DNA complexes from cultured chick embryo fibroblast cells have been separated by agarose gel chromatography at intervals after pulse labelling with [ 3 H]thymidine. 2. 2. The proportion of the various cytosol components changed markedly with time: there was a lag period of 3 hr before the major labelled (5 x 10 5 dalton) DNA complex appeared in the cytosol, and a further lag period of 5 hr before it was extruded from the cell. 3. 3. Cultured chick embryo fibroblast, and rat spleen, cells rapidly and very efficiently import their own or each others cytosolic DNA complexes into their respective cytosol fractions: the material recovered from the cytosol of recipient cells is characteristic of the presented material. 4. 4. Homologous cytosolic DNA complex presented to chick embryo fibroblast cells also becomes associated with the nucleus. The rat at which this occurs is comparable with the rate of incorporation of [ 3 H]thymidine into nuclear DNA.
