To identify a toxin and its producer isolated from woody material in a building where the occupants experienced serious ill health symptoms. Hyphal extracts of an indoor fungus, identified as the cycloheximide-tolerant species Acremonium exuviarum, inhibited motility of boar spermatozoa (EC50 5 +/- 2 mu g of crude solids ml(-1)) and caused cytolysis of murine neuroblastoma cells (MNA) and feline fetal lung cells (FL). The responsible substances were purified and identified as two structurally similar, heat-stable, novel, toxic peptaibols, 1726 Da and 1740 Da, respectively, with amino acid sequences of Acetyl-Phe-Iva/Val-Gln-Aib-Ile-Thr-Leu-Aib-Pro-Aib-Gln-Pro-Aib-(X-X-X)-S erOH and Acetyl-Phe-Iva/Val-Gln-Aib-Ile-Thr-Leu-Val-Pro-Aib-Gln-Pro-Aib-(X-X-X)-S erOH. Purified acrebol inhibited motility of boar sperm, depleted ATP half-content in 1 day (EC50 of 0.1 mu g ml(-1), 60 nmol l(-1)) depolarised the mitochondria after 2 days, but did not affect the cellular content in NADH. This indicates mitochondrial toxicity. Plate-grown biomass of A. exuviarum BMB4 contained 0.1-1% (w/w) of acrebol, depending on the culture medium. Acrebol paralysed the energy generation of mammalian cells suggesting that mitochondria were its target of action. Acremonium exuviarum, as an indoor fungus, is potentially hazardous to health because of the toxic peptaibols that it produces.