One hundred and fifty-two fungal isolates from the air spores of 503 damp, mouldy dwellings in Glasgow and Edinburgh, Scotland, were screened for cytotoxicity against the human embryonic diploid fibroblast lung cell line, MRC-5 (ATTC No. CCL171). At least 37% of the isolates, principally of the genus Pénicillium, showed toxicity to the cells when subjected to the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay following a water extraction procedure. When spores of 23 of the isolates were subjected to extraction with the organic solvent dimethyl sulphoxide, an additional level of cytotoxicity was observed separate from the water extraction methods. Many mycotoxins are known to be sparingly soluble in water. When some of these isolate extracts were exposed to human Chang liver cells (ATCC No. CCL13) and to human normal sternal bone marrow cells, (Detroit 98, ATCC No. CCL18), some additional cytotoxicity was observed. A comparison was also made, in some cases, between results obtained in the MTT and neutral red assays. Organisms exhibiting the greatest cytotoxicity included Pénicillium viridicatum, P. expansum, P. chrysogenum, P. commune and Aspergillus fumigatus.