Sephacryl S-300 gel filtration of animal sera is used to demonstrate that there are at least 2 components that promote the 'spreading' of cells in culture. A morphometric technique is described to quantitate the spreading process. For a number of cell strains and established cell lines the more quantitatively significant spreading factor is not fibronectin. Rather it is a component with fibronectin and seems to cause spreading via a different mechanism from that stimulated by fibronectin. Thus

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... ibronectin will cause spreading in the absence of protein synthesis, whereas the smaller component requires protein synthesis. The kinetics of spreading are also different at all concentrations of the factors that are effective. By comparing the spreading promoted by whole sera with that promoted by separate serum fractions following chromatography we conclude that under normal conditions plasma fibronectin plays little part in initial cell spreading. This view is supported by the fact that fibronectin-depleted serum will stimulate cell spreading.