Cell-free extracts of soybean callus and pea root tissue catalyzed the reduction of GDP to dGDP. GDP reduction by these extracts is believed to be catalyzed by ribonucleotide reductase. Crude extracts of soybean callus and pea roots catalyzed the formation of 19.0 and 0.29 nmoles of dGDP per mg protein per hour. Ribonucleotide reductase of soybean callus was purified by ammonium sulfate precipitation and Sephadex G-50 and DEAE-cellulose chromatography. Specific enzyme activity was increased approximately 7-fold. Reductase activity in a partially purified extract responded to increased amounts of protein and substrate, and increased incubation periods. Enzyme activity was not enhanced by Fe++, ATP, Mg++ or B12 coenzyme. Low concentrations of dithiothreitol and dTTP, however, stimulated reductase activity. The soybean callus and pea root extract also contained an undetermined amount of phosphatase, nucleosidase and deaminase activity.