Survey of twelve mouse tissues revealed the presence of appreciable phenylalanine hydroxylase activity in the pancreas and kidney as well as the liver but in no other of the tissues tested. Single cell suspensions of mouse liver were prepared by use of tetraphenylboron. The enzyme activity of such suspensions was much more stable than that of liver extracts, and permitted determination of the Michaelis‐Menten constant, the pseudo‐first order reaction velocity constant on a cell‐number basis, and the temperature coefficient and apparent activation energy of the enzyme activity. Possible applications of these methods to problems in cellular biology have been indicated. Copyright © 1969 Wiley‐Liss, Inc.