1. 1. The conversion of the insecticide aldrin to its epoxide dieldrin has been found to be much lower in trout liver microsomes (TLM) than in male rat liver microsomes (MRLM). 2. 2. The low epoxidative activity in TLM is not due to inadequate incubation conditions or to the presence of endogenous inhibitors, but rather is due to a deficiency of the epoxidative enzyme. 3. 3. When liver microsomes of trout and male rat are mixed, the epoxidative activity is greater than expected, the enhancement being two- to fourfold. 4. 4. The enhancement in the combined system of TLM and MRLM is the result of an interaction between the microsomal electron transport systems of TLM and MRLM wherein the component enzymes, especially electron transport component “X” in TLM and epoxidative “E” in MRLM, are more fully utilized.