THIS communication contains the results of one part of the research programme aimed at studying the effect of magnetic fields on living organisms and tissues. Female C3H strain mice from the Roscoe B. Jackson Memorial Laboratory were kept for 4–5 weeks in magnetic fields of 4,200 gauss strength, produced by ‘Alnico’ permanent magnets. The animals were placed in individual cages 3.5 in. inner diameter and 1.40 in. height. The magnetic field-strength was in the centre of the cages 4,400 gauss, decreasing to 4,000 gauss toward the periphery of the cage, as measured with a Dyne-Empire Model D-79 gauss meter. Control animals were kept in identical cages, wherein the pole-pieces of the magnets were simulated with 1-in. thick brass disks to provide identical conditions with respect to temperature, illumination and cosmic radiation1. The food of the animals consisted of standard Rockland mouse diet in pellet form. Blood samples taken from the tail at intervals of 7–14 days were investigated with respect to white blood count, red blood count and differential count.