Sporidesmin, a fungal toxin with widespread distribution within New Zealand, was previously shown to be a potent clastogen in Chinese hamster cells in vitro, but not in peripheral blood lymphocytes of sheep in vivo. In mice, massive oral doses led only to slight increases in micronucleus levels in the bone marrow, despite highly significant changes to other toxicological parameters. It would appear that the intact animal is protected in some fashion from the clastogenic effects of sporidesmin, although this substance must still be considered a potential human genotoxic agent.