Two naturally occurring fungal mycotoxins, sterigmatocystin and griseofulvin, were tested for induction of sister-chromatid exchanges (SCEs) in bone marrow cells of female Swiss albino mice. Sterigmatocystin gave elevated SCE frequencies at all doses tested (0.06-6.0 mg/kg). In contrast, griseofulvin, tested from 0.4 to 200 mg/kg, elevated the SCE frequency only in those mice which received doses of 100 or 200 mg/kg body weight. These results indicate that both fungal mycotoxins induce SCE in vivo and are potentially mutagenic.