os Se -J- baie of two isotopes of plutonium is seriously inconsistent with the negligible influence of isotope effects on the chemical «inetics of heavy elements. Thus it seems necessary to explain this apparent solubility difference on physical grounds. The specific activity of the 75° Pu0, particles (~80% **°Puo, and ~20% **°Pu0,) was about 220 times that of **°Pu0,. In addition the *3*Pu0, particles exhibited a very significantly lower density than the *3°Pu0, particles (30) , indicating a highly faulted structure and weakened intermolecular bonding for the 23°Pud, particles. Fleischer 3) proposes that the apparently higher dissolution rate for 23° Pud, may be explained by the alpha recoil nucleus ablation of the surface iayers of the particles, with a fragmentation rate proporticnal to the specific alpha disintegration rate and with variable sizes of fragments ranging up to ~10° atoms. The poorer structural integrity of the 75° Pu0, particles may give rise to an. --dncrease in the size range of the ejected fragments. Such small fragments, ranging pp to tens of angstroms in diameter or more, would pass readily through the 0.1 um diameter pores of the membrane filters used in the dissolution experiments 2”), Also, such small ablation fragments may exhibit a much higher mobility in tissue than that of 0.1 to 1.0m diameter, the eizc range of particles used in most animal inhalation experiments. This greater mobility for very small ablation fragments in tissue may explain the observed more rapid rate of translocation for 73° Pud, than for ***Pu0, from the lung tc the liver and bone (32233). Another explanation for the apparently higher solubility of 29PW0, than ?°*Pu0, is the possibility that the intense alpha radiolysis of the lung fluid at the surface of the particles leads to the production of ehemically active free radicals which in turn react with Pud, molecules on the particle surface. This process also would proceed at a rate proporticnal to specific activity and to particle surface area, Ina this ue