The suggestion by Fersht [(1974) Proc. R. Soc. London Ser. B 187, 397-407] that enzymes that provide maximal rates of catalysis should be characterized by values of Ks, the dissociation constant of the enzyme-substrate complex, greater than 10 times the value of the ambient substrate concentration has been examined. 2. For such enzymes, Ks is not relevant, and attention is best focused on the relative numerical values of k(cat). (in units of s(-1) and the substrate molarity. It is necessary only that the former be about 10(10)-10(11) times the latter to ensure that the rate of product formation be diffusion-limited and thus maximal.