Seventeen laboratories participated in an interlaboratory study conducted to provide precision and accuracy statements for the proposed EPA Method 605 for measuring concentrations of the Category 7 chemicals benzidine and 3,3'dichlorobenzidine (DCB) in municipal and industrial aqueous discharges. Method 605 involves extraction of benzidine and DCB from the aqueous sample at pH 7-8 with chloroform. The extract is then back extracted into acid, re-extracted into chloroform at neutral pH, and concentrated. The benzidines are determined in the final extract using high performance liquid chromatography (HPLC) with electrochemical detection. The study design was based on Youden's plan for collaborative tests of analytical methods. The resulting data were statistically analyzed using the computer program 'Interlaboratory Method Validation Study' (IMVS). Mean recoveries of benzidine and DCB ranged from 48-70 percent. Overall precision ranged from 38-69 percent and single-analyst precision ranged from 24-40 percent. A statistically significant effect due to water type was established for DCB in the case of one wastewater. However, review of the precision statistics indicated the effect was of no practical importance. There were no other discernible differences due to water types among mean recoveries, overall precisions or single-analyst precision.