An interlaboratory study in which 18 laboratories participated was conducted to provide precision and accuracy statements for the proposed EPA Method 609 for measuring concentrations of the Category 4 chemicals nitrobenzene, isophorone, 2,4-dinitrotoluene and 2,6-dinitrotoluene in municipal and industrial aqueous discharges. Method 609 involves solvent extraction of the pollutants with methylene chloride followed by Florisil clean-up and subsequent gas chromatographic analysis of the four subject compounds using flame ionization and electron capture detection techniques. The study design was based on Youden's plan for collaborative tests of analytical methods. Three Youden pair samples of the test compounds were spiked into six types of test waters and then analyzed. The test waters were distilled water, tap water, a surface water, and three different industrial wastewater effluents. The resulting data were statistically analyzed using the computer program entitled 'Interlaboratory Method Validation Study' (IMVS).