FIG. 3.   Primary keratinocytes are more sensitive to N-9 than to C31G or SDS during long-term exposure. Primary human vaginal keratinocytes were exposed to N-9, C31G, or SDS for 48 h and subsequently assessed for viability using MTT assays. (A) Cell survival following exposure to concentrations of 2.5 × 10⁴ to 5 × 10³%; (B) Cell survival following exposure to N-9 at concentrations of 2.5 × 10⁵ to 2.5 × 10⁴%. The arrows in panels A and B both indicate cell viability at 2.5 × 10⁴%. Cell viability following microbicide exposure is expressed as the fraction of viable cells relative to the number of mock-exposed cells. The results illustrated are averages from two experiments in which triplicate wells for each concentration were assayed.