Three animals were used for each species and gender, with the exception of mouse (six animals) and monkey (two). Livers were homogenized in 0.01 m Tris-HCl}0.14 m KCl, pH 7.4 buffer (1:3 w}w) at 04 C. Microsomes were prepared by ultracentrifugation (100000 g for 75 min) of the post-mitochondrial supernatant (10000 g for 20 min). The microsomal pellet was resuspended in homogenization buffer and centrifuged for
60 min at 100000 g. The washed microsomes were suspended in homogenization buffer (weight equal to the amount of liver homogenized) and stored at approximately 80 C. Pooled microsomes containedequal amounts of the individual microsomal preparations.Liver microsomes prepared from mouse, rat, dog, micropig and monkey were from untreated animals. Rabbit liver microsomes were from animals used for acute dermal irritation studies, in which no signicant dermal effects were found.