Improved media have reduced the amount of serum protein required for clonal growth of normal human and chicken fibroblast-like cells. In the presence of limiting amounts of serum protein, attachment of colonies to tissue culture plastic surfaces is weak. Treatment of the culture surface with polylysine or other basic polymers causes the cells to adhere much more tightly. Growth is also improved on the surfaces treated with basic polymers, and further reductions in the concentration of serum as possible. At sufficiently low protein concentrations, growth of some types of cells is totally dependent on the use of a treated surface. Several different types of normal human and chicken fibroblast-like cells show improved growth on polylysine-coated surfaces, but no improvement was obtained in growth of a line of SV-40 transformed WI-38 cells. Acidic and neutral polymers are generally inactive. Collagen and gelatin improve growth slightly, but the effect is much less than that obtained with basic polymers. Both natural and synthetic polymers with an excess of basic groups are active, including histone, polyarginine, polyhistidine, polylysine, polyornithine, and protamine. The only critical requirement appears to be a polymer that carries a positive charge at a physiological pH.

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