This study, the first for quantitative determination of DDT in milk, was designed to permit evaluation of the precision and accuracy obtainable by using the AOAC Official Method for the Colorimetric Determination of DDT Residues, and to obtain data which would be helpful in planning future studies for the evaluation of other methods. At the request of the Analytical Reference Service, all participants were volunteer agencies having responsibilities in the field of milk and food contamination. Provision was made for the participants to critique the procedure.