Plutonium-239 was injected into 12-week-old female CBA/H mice in the range 1.85-18.5 kBq kg-1 either as a single injection or as 16 injections spaced at 3.5 day intervals over eight weeks. There was a highly significant increase in the yield of fully developed osteosarcomas with increased amounts of 239Pu for both modes of injection. Osteosarcomas too small to be diagnosed radiographically were also seen in many bones and small but significant yields of myeloid leukaemia were seen in animals given plutonium. Although more myeloid leukaemia was seen in the mice given plutonium in divided amounts than in those given the plutonium in a single injection it could not be shown that multiple injection significantly affected the yield of either late effect.