Fumonisins B1 and B2 and AAL toxin are a series of structurally related mycotoxins. Fumonisins B1 and B2, produced by Fusarium moniliforme Sheldon induce toxic hepatitis and hepatomas in rats and leukoencephalomalacia in horses. The cancer-promotion assay which has been used to guide their purification is slow and consumes large amounts of sample. We have examined a series of cultured mammalian cell lines in order to develop a more rapid and sensitive bioassay system, which may be useful for examining structure-activity relationships and the mechanism(s) of action of these toxins. Of 9 rat hepatoma cell lines tested, all except the two most de-differentiated line were sensitive to the three toxins, with a toxic response visible by 48 h. Approximate IC50 values for the most sensitive hepatoma line, H4TG, were 4, 2 and 10 micrograms/ml for fumonisins B1, B2 and AAL toxin, respectively, in 100 microliters cultures. Among 15 cell lines from other sources, only MDCK dog kidney epithelial cells were sensitive (IC50 = 2.5, 2 and 5 micrograms/ml, respectively). Studies in co-cultures of sensitive and insensitive cell lines and in cultures of a sensitive cell line over a range of cell densities indicated that cytotoxicity of fumonisins B1 and B2 does not involve metabolite activation to a derivative stable enough to diffuse to adjacent cells.