Specific bioassay methods for certain insecticides can be developed by the integration of physico-chemical and biological techniques. Based on high insect toxicity, short exposure time, synergistic effect of sesamex, solvent-water partition, and alkali stability, a specific bioassay method was developed for Bidrin® insecticide (3-hydroxy-N,N-dimethyl-cis-croton-amide dimethyl phosphate). The overall procedure was formulated by the calculation of the effects of each factor on the recovery of Bidrin and the interference of other insecticides. The mean recovery for Bidrin was 96.0%. Ppm of Bildrin can be detected as low as 0.05 in extracts after cleanup. The interference of any of 26 organophosphorus, 3 carbamate, and 17 chlorinated insecticides tested is less than 6%; most of them less than 1%. Insecticides which are very soluble in water, such as dimethoate (5.9% interference), trichlorfon (3.5%), phosphamidon (2.5%), mevinphos (1.2%), and Isolan® (1-isopropyl-3-methyl-5-pyrazolyl dimethylcarbamate) (1.6%), gave relatively higher interferences. In most cases, determined and calculated values are in agreement. From this study several new synergistic combinations were found.