Acid cholesterol esterase in the 9000 g precipitates of rat liver was stimulated by the addition of organophosphate pesticide, dimethyl dichlorovinyl phosphate (DDVP) in vitro. The major portion of the esterase in the 9000 g precipitates may be lysosomal esterase. The esterase activity was also stimulated by the osmotic treatment of the 9000 g precipitates as well as that in the presence of DDVP. However, the esterase activities in the osmotic treated-9000 g precipitates and the supernatant solution obtained by centrifugation after the osmotic treatment of the 9000 g precipitates were no longer stimulated by the addition of DDVP. These results suggest that the stimulatory effect of DDVP may be due to the release of the esterase by the disruption of lysosomal membrane or the translocation of the esterase on lysosomal membrane from a bound type a free one.