This is the report of a symposium held at the Wistar Institute of Anatomy and Biology in Philadelphia on March 16, 1967. Fifty-four participants (four from outside the United States) discuss factors concerned with growth of animal cells in culture. Inhibitors such as Bullough's chalones and Szent-Gyorgyis, retine, and the cell growth promoter, promine, and other regulators of cell growth are discussed. The authors admit the difficulty of relating reaction of in vitro systems to in vivo situations. Lively discussions follow chapters. The symposium will be of interest to biologists, cancer chemotherapists, and those interested in the kinetics of cell growth.