2,4-Pentanedione was evaluated for the ability to increase the incidence of micronucleated polychromatic erythrocytes in bone marrow of male and female Swiss-Webster mice treated by single i.p. injection (Micronucleus Test). Groups of at least 10 mice (at least 5 males and 5 females) were injected with 2,4-pentanedione in water vehicle at doses of 0, 200, 400 or 650 mg/kg (high-dose = approximately 89% of the LD50). The peripheral blood was sampled for micronucleus evaluation 30, 48, and 72 hrs after dosing. The ratio of normochromatic to polychromatic erythrocytes was not significantly different (p > 0.05, Fisher's Exact Test) inthe treated groups compared with the vehicle controls. Statistically significant increases in the numbers of micronuclei were observed at the 30-hr sampling time for male and female mice at the mid- and high-dose levels (p < or = 0.001) and at the 48-hr sampling for the combined sexes at all treatment levels (low-dose level, p < 0.05, and mid- and high-dose, p < or = 0.001).