case the dissolved plutonium would diffuse away from the hot particles. Eowever this dissolved piutoniun undoubtedly would be slowly redistributed in the lung in the same fashion as that reported by Moskalev (34) for inhaled soluble compounds of plutonium, resulting in a highly non-uniform distribution, with hot spots located preominantly in the sub-pleural region of the lungs. This gradual conversion of the soluble plutonium compounds to small colloidal size particles at focal points of activity may be the result of the self-chelating properties of tetravalent plutonium in solution. -In recent studies of rat inhalation of *°°Pu0,» Sanders (11) has ¢ demonstrated a substantially increased risk per rad for small lung burdens of aged, "crushed" 7°*Pu0, 2 microspheres. In this case the inhalec particles involve smaller particles and a correspondingly larger surface area. The observed more rapid rate of tiauslucation to other organs can variously to the higher mobility of the smaller particles, or to the higher rate of surface ablation (or dissolution) for the increased surface area, or both. The higher tumor incidence can be attributed to the fact that- ‘the greater mobility and wider redistribution of the ***Pu0, ‘microspheres and their breakdown products subject a much larger number of cells to a limited number vf alpha interactions. 7 The correctnéss of the above interpretation is reinforced by the results of the Los Alamos ceramic sphere experiments reported by Richmond et al. (12,13) and further discussed by Bair et a1, °19) . In these experi- ments 2000 Zirconium oxide microspheres of 10 pm diameter, each set containing a specified amount of plutonium, were injected into the lungs of groupsof experimental animals. The total pluteniun per microsphere ranged from 0.07 to 1.6 pCi of 7°*Pu and from 4.3 to 59.4 pCi of **#Pu, with identical activity for each of che 2000 microspheres in each of eight animal exposure groups of 70 animals ner group. The local dose rate,