FIG. 4.   Repetitive microbicide exposure affects the viability of human vaginal keratinocytes. Primary human vaginal keratinocytes were exposed to N-9, C31G, or SDS for 2 h. The cells were then washed once with PBS and cultured under new medium for 24 h. This cycle of exposure and recovery was repeated two more times. Cell survival was assessed using MTT assays following the third exposure/recovery cycle. 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.