l h e osmoprotectant glycine betaine is synthesized via the pathway choline -+ betaine aldehyde + glycine betaine. I n spinach (Spinacia oleracea), the first step is catalyzed by choline monooxygenase (CMO), and the second is catalyzed by betaine aldehyde dehydrogenase. Because betaine aldehyde is unstable and not easily detected, we developed a coupled radiometric assay for CMO.['4C]Choline is used as substrate; NAD+ and betaine aldehyde dehydrogenase prepared from Escherichia coli are added to oxidize [l4C1betaine aldehyde to ['4Clglycine betaine, which is isolated by ion exchange. The assay was used i n the purification of CMO from leaves of salinized spinach. The 1 O-step procedure included polyethylene glycol precipitation, polyethyleneimine precipitation, hydrophobic interaction, anion exchange on choline-Sepharose, dimethyldiethanolamine-Sepharose, and Mono Q, hydroxyapatite, gel filtration, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Following gel filtration, overall purification was about CMO is a homodimer. CMO preparations were red-brown, showed absorption maxima at 329 and 459 nm, and lost color upon dithionite addition, suggesting that CMO is an iron-sulfur protein.