In this study, two solutions of ethyl acetate containing different mixtures of lindane, heptachlor epoxide, DDE, and dieldrin were analyzed, both before and after dilution with water, by 31 participating laboratories. One participant used paper chromatography, and the rest used gas chromatography; four of the latter employed the microcoulometric detector, and the rest used the electron capture detector. Best resus were obtained with a mixed column packing containing both polar and nonpolar materials. Satisfactory results were also obtained with a single-component column when a prior separation of the pesticides by passage through a florisil or alumina chromatographic column was made before injection on the gas chromatograph. The newer model C-200 Dohrmann microcoulometer produced satisfactory results at the concentrations present in this sample. Failure to correct adequately for incomplete recovery of pesticide in the extraction procedure produced low results in the diluted sample.