In a previous paper, we showed that a cell line derived from hemocytes of the cabbage armyworm, Mamestra brassicae (R-cell) was a thousand times as resistant to rotenone as that from ovaries of the same species (S-cell). The S-cells were killed by rotenone at concentrations higher than 10(-9) M, while R-cells at higher than 10(-6) M. When the R-cells were cultured in the medium containing 10(-9) M rotenone, the ability of rotenone to kill the S-cells was lost in the used medium. Also, when rotenone was incubated in the medium conditioned with R-cells, it lost its cell killing activity. It became evident that rotenone-inactivating substance(s) were produced in cells and stored in water-soluble form or liberated into the medium. The substance(s) were inactivated by heat treatment.