The work on iodinated proteins, described in the present symposium of papers, was directed mainly towards large-scale experiments on the stimulation of lactation in cows. However, it became evident at an early stage that iodine analyses did not give a quantitative indication of the biological activity of the artificially iodinated proteins, and that tests on laboratory animals would be required before the main work on cows could be undertaken. In the first place, some use was made of the test based on changes in the oxygen consumption of guinea-pigs, determined by the method of Richards & Collison [1928]. This test is lengthy and laborious, and was of little value under the existing conditions for obtaining quantitative results on a large number of preparations. Other methods, reviewed in part by Burn [1937] and by Wokes [1935], were therefore considered. Determination of changes in the metabolic rate by estimation of the carbon