The effect of dietary fatty acids on the spontaneous autoimmune disease systemic lupus erythematosus of the MRL-mp-lpr/lpr mouse was investigated after onset of disease. Parameters monitored included auto-antibody levels (by ELISA), proteinuria, body weights, clinical appearance and survival. The preliminary study (using five mice per group) showed that dietary restriction improved survival (P less than 0.001) per se, and further that all mice receiving the fish oil diet benefited considerably (P less than 0.005), whilst males on the fish oil/evening primrose oil diet also lived longer than expected (P less than 0.005).