The procedure described in the U.S. Atomic Energy Commission Regulatory Guide 4.3, which is intended to keep 131I levels in milk “as low as practicable, ” does not properly account for all sources of analytical error that can occur in a low-level radiochemical procedure. Inaccurate chemical recovery factors due to large amounts of stable iodine in milk and incomplete evaluation of detector background fluctuations are the sources of largest error. Significant errors are also contributed in incorrect application of factors for protein-bound iodine and counting efficiency. Ambient levels of 131I contribute to difficulties in evaluating the local impact of any reactor. Use of a beta/gamma coincidence system for counting 131I, measurement of the stable iodine in milk samples and collection of control samples remote from any site are among the recommendations provided to ensure compliance with the Regulatory Guides.