The target molecular size of the red beet (Beta vulgaris L. var. Detroit Dark Red) plasma membrane Ca 2 + -ATPase was determined by radiation inactivation analysis involving γ-rays from a 6 0 Co source. The Ca 2 + transport activity associated with the Ca 2 + -ATPase in native plasma membrane vesicles declined in an exponential manner with radiation dose and the target molecular size was estimated to be about 245 000 ± 5000. As the catalytic subunit (phosphorylated intermediate) of the red beet plasma membrane Ca 2 + -ATPase has been shown to be about 100 000 to 120 000, these results suggest that the enzyme exists as a dimer in the native membrane.