AbstractC57BL/Ka mice were fed diets rich inω3 orω6 polyunsaturated fatty acids and inoculated with the thymic lymphoma‐inducing retrovirus RadLV. Mice receiving theω3 diet died significantly sooner than those receiving theω6 diet. Of the three known mechanisms by which fatty acids can exert antiviral activity, namely, modification of membrane fluidity, modulation of immune response, and synthesis of metabolites with antiviral activity, the first two can be eliminated in the model under study. It is therefore concluded that differences in survival are due to fatty acid metabolites with distinct antiviral activities.