At the present time there is an increasing awareness of the potential harmful effects of various chemical food additives used to delay spoilage. This resulted in the search for natural substances which may prevent oxidation of various lipids, a main cause of the onset of rancidity. Various algae were examined for their potential as sources of antioxidants. As part of that search, the amount of doublebonds in their constituent compounds and their antioxidant (AO) and lipoxygenase (LOX) activities were followed in some microalgal species. The effect of light intensity, salinity, and temperature on the AO activity of lipid extracts from the microalga Navicula sp. was studied. Maximum AO activity was obtained in Navicula grown under low light intensity. AO activity of various algal fractions was studied both as inhibition of LOX activity and of linoleate autooxidation. Most of the LOX inhibitive activity was found in the membranal lipid-soluble fractions, whereas the inhibition of autooxidation was associated with the water-soluble components of the cell cytoplasm.