Extracts of over 100 species of microalgae both from nature and from laboratory cultures were screened for antioxidant activity. As an assay for antioxidant function, we examined the inhibition of the activity of two oxidizing enzymes, lipoxygenase and tyrosinase. Water, ethanol, and methanol extracts of microalgae were used for the assay. It was found that lipoxygenase and tyrosinase activities were inhibited by the extract of several microalgae. Our results suggest that these species of microalgae have useful antioxidant activity, and they will be further examined for potential biotechnological exploitation. The aim of our screening was to identify potential sources of natural antioxidant compounds, which if found to be sufficiently potent and nontoxic, might eventually replace the chemicals that are currently used as food additives and cosmetics, in order to prevent food spoilage and their oxidation.