Plasma membrane vesicles from pea (Pisum sativum L.) seedlings contain an autophosphorylating calcium-activated protein kinase of relative molecular weight 18,000 (phosphoprotein 18, pp 18). Pulse chase analysis revealed that pp 18 autophosphorylation exhibited very rapid turnover. pp 18 was the only detectable plasma membrane protein to have this property. pp 18 has been highly purified by affinity chromatography and the final preparation contains peptides of relative molecular weight 67,000, 48,000, and 18,000. Highly purified pp 18 still showed rapid cycling of autophosphorylated phosphate attached to pp 18. Turnover of pp 18 autophosphorylation was accelerated by ADP, but this effect did not appear to be the back reaction of the kinase since inorganic phosphate accumulation is increased by ADP. A rapid cycling of phosphorylation is an ideal control point in signal transduction.