1. Experiments were carried out during the years 1931–5 in which mice on a basal diet of raw milk and white-flour biscuit were compared with mice on a similar diet except that the raw milk was replaced by commercially holderpasteurized (145–150° F. for 30 min.) milk belonging to the same batch. Each group of mice comprised in aggregate about 1100 animals, which were usually 4–6 weeks old at the commencement of observation. For reasons given on p. 206 no attempt was made to standardize the volume of milk consumed by the individual mice. Except in the preliminary experiments, when the milk was given in excess, the amount supplied was regulated more or less in accordance with the requirements of the average animal.