Plasma-activated medium (PAM) has a broad prospect in the medical field. However, how to define the plasma dose of PAM and what is the dose-response relationship of PAM on cells are still open questions of plasma medicine. In this paper, the plasma dose of PAM based on equivalent total oxidation potential (ETOP) is introduced, and the S-logistic model is used to describe the relationship between PAM dose and the corresponding lethal effects of cells. Experiments of PAM on A875/HaCaT cell viabilities indicate that ETOP as a plasma dose is suitable for PAM. Evidence of dose discrepancies in 50% response intensity suggests that ETOP can be used to maximize the lethality difference between normal/cancer cells. Further validation by the published literature again indicates that ETOP may provide a well-defined strategy in evaluating the selectivity of PAM treatment on different cell types. Published under an exclusive license by AIP Publishing.