1. DMC has no insecticidal properties but markedly enhances the effectiveness of DDT against resistant house flies. The addition of a small amount of DMC causes a significant drop in the amount of DDT required for a given mortality.

2. Data interpolated from probit-log dosage lines for DDT alone and DDT plus DMC show that, under experimental laboratory conditions, the resistance of the Roberds strain can be markedly reduced.

3. Greatest synergistic effect is manifested when DMC and DDT are applied together, for separate application of the chemicals at 6-hour and 24-hour intervals shows a marked reduction in mortality.

4. DMC is rapidly metabolized by living flies and is excreted principally as a product tentatively identified as bis-(p-chlorophenyl) acetic acid (DDA). The compound 1,l-bis-(p-chlorophenyl) ethylene is suggested as being an intermediate product in DMC metabolism.

5. A correlation is shown between the extent of synergistic action and the degree of inhibition of DDT-detoxification in resistant flies.

6. Inhibition appears to be of a competitive type, the synergist competing with the insecticide for the mechanism of detoxification.