The Ca2+-pumping ATPase of plasma membrane, known to exist since 1966 [13], has now been characterized as a high Ca2 + affinity enzyme, present in all likelihood in all eukaryotic plasma membranes (for a recent review, see Ref. 11). The essential properties of this enzyme, as they can be extracted from a very large number of studies, are summarized in Table I. The ATPase is perhaps less well characterized than other members of the E I, E2 class of transport ATPases like the Na + IK + -ATPase and the Ca2 + -ATPase of sarcoplasmic reticulum. For example, it is still not known whether the acyl phosphate formed during the enzyme cycle is an aspartyl phosphate. It is also not established to the satisfaction of all specialists whether the Ca2 + I ATP stoichiometry is, as indicated in Table I, always and invariably 1. Recent reviews on the ATPase have appeared, the most comprehensive being perhaps the one by Schatzmann.