Sullivan  has stated without reservation that our paper [Tolmasoff et al., 1980] did not present data supporting the possibility that superoxide dismutase has a role in determining longevity in mammalian species. He states that the data instead support the hypothesis that there is no association between superoxide dismutase and lifespan. On reading his arguments, I find no errors in our data analysis or in the conclusion made. Instead, I find he has failed to recognize the critical components of our data and has consequently misinterpreted the results. Because similar oversight might be made by other readers of this paper, I have included a brief review of the rationale underlying our investigation of longevity determinants in mammalian species along with the reply to Sullivan’s criticisms.