Chitosan is less toxic to human cells than benzoyl peroxide. Human monocytes (A) or keratinocytes (B) were incubated with chitosan or benzoyl peroxide for 16 hours. Cell viability was then determined by MTS assay. Error bars represent SEM ( n = 3, independent experiments for A, one experiment for B). **p < 0.01, and ***p < 0.001 (t-test).