The enhanced acute toxicity of 239Pu as compared on an equivalent microcurie basis with 238pu was evidenced by mortality data, by weight loss, by hematological and by histopathological observations. This difference in toxicity would seem to be most reasonably explained in terms of the different patterns of internal deposition of the 2 isotopes, which, in turn, may be explained by differences in physical state of the isotopes resulting from the 270-fold greater mass associated with an equal-microcurie quantity of 239Pu. No evidence was seen of direct chemical toxicity of 239Pu.