[en] Cross-placental transfer of ²³⁹Pu from the mother to fetus was studied in C3H mice. The activity of ²³⁹Pu in the conceptus was measured 24 hrs after the intravenous injection of ²³⁹Pu citrate on days 10.5 to 16.5 of gestation. More ²³⁹Pu was transferred to the conceptus when the plutonium was administered in the later stages of gestation; 0.12% of the injected dose per conceptus on day 10.5 and 1.3% on day 16.5. In all the gestational stages examined, the yolk sac and decidua contained more than 89% of the total activity distributed in the conceptus. The concentration of ²³⁹Pu in the yolk sac was about two orders of magnitude greater than that in the fetus. The ²³⁹Pu concentration in the maternal liver decreased with the gestational stage. In the early gestational stages the concentration in the maternal liver was greater than that in the yolk sac; but, this relationship was reversed in the later stages. (author)