1. Illudin S, a toxic principle of the basidiomycete Lampteromyces japonicus, was incubated with rat liver 9000 g supernatant and its metabolites studied. 2. Two metabolites, M1 and M2, were isolated and identified as cyclopropane ring-cleavage compounds by n.m.r., i.r. and mass spectral analyses. Moreover, M2 contained a chlorine atom. 3. On the basis of detailed analyses of the 2D n.m.r. spectra and differential nuclear Overhauser effect experiments, the previous assignments of the cyclopropane carbons of illudin S were revised.