Cell spreading and cell division rates of a transformed line of mouse lung fibroblasts were studied on various polymer surfaces in the presence of serum proteins. The bare polymer surfaces, as well as the protein-coated surfaces, were characterized by their polar (gamma ps) and dispersion (gamma ds) surface free energies. Cell spreading appeared to be dependent on the polar surface free energy. Cell spreading is low when the gamma ps of the bare surface is lower than 5 erg cm-2; marked spreading occurs when gamma ps is higher than 15 erg cm-2. A similar relationship was found between cell spreading and polarity of the protein-coated surfaces, although less pronounced than for the bare surfaces. Cell spreading appeared independent of the dispersion surface free energy. Cell division rate was the same on all surfaces tested.