Ascorbate oxidase activity and ascorbic acid content were followed during the development of muskmelon (Cucumis melo L. var. reticulatus) fruits. The enzyme was highly expressed in ovaries and very young fruit tissues, followed by a decrease in 10- and 20-d-old fruits and an increase in 30- and 35-d-old fruits which coincided with early events of fruit ripening. Ascorbic acid content was negatively correlated with ascorbate oxidase activity. The enzyme was purified to homogeneity following ion exchange, affinity and gel filtration chromatographic trials. The purified enzyme was a glycoprotein of molecular weight 137 000 composed of two subunits of molecular weight 68000, and formed by six isoenzymes with isoelectric points in the range of pH 7.7 to 8.3. Its electron paramagnetic resonance and optical spectra were in agreement with other copper proteins and the enzyme contained eight copper atoms per dimeric molecule. The Km of the enzyme for ascorbic acid was 50 μM. Ascorbate oxidase activity was inhibited by azide and by EDTA, two inhibitors of copper proteins. Optimal conditions for enzyme activity was pH 5.5, and a temperature of 37 °C. Polyclonal antibodies were produced against the purified protein and immunoprecipitated ascorbate oxidase activity.