Determination of ribonucleotide reductase (EC 1.17.4.1) activity in plant extracts requires the optimum resolution of substrate and product in the radioactive enzyme assay. By ion exclusion or ion-exchange chromatography of nucleoside 5'monophosphates or nucleosides, respectively, on cation-exchange resins (e.g., Dowex 50, Beckman M 71) it is possible to reproducibly measure 0.1% product (deoxyribose derivative) formation, besides a large excess of unreacted substrate (ribonucleotide). Use of a nucleoside analyzer reduces the separation time to 2-3 hr, compared with at least 18 hr in regular column chromatography. A new, fast, and simple method of product estimation at the nucleoside level is described. Labeled deoxycytidine or deoxyadenosine are separated from cytidine or adenosine, respectively, by passage of the dephosphorylated sample over small columns of anion exchanger, OH-form (e.g.. Dowex I) in 50% methanol; the alcoholic eluate is combined with a dioxane-based scintillation mixture, and radioactivity is recorded directly in a continuous-flow liquid scintillation counter. Ribonucleotide reductase was demonstrated in root tips of germinating broad bean (Vicia faba) and in green algae. II9