In recent years interest in mycotoxin-producing fungi growing in indoor environments has increased. Evidence of mutagenic potential of some molds or their mycotoxins is still equivocal. Much more information is available for single mycotoxins or their metabolites than for complex mixtures composed of different fungi products. The aim of this study was to identify the mutagenic potential of extracts isolated from mold-attacked buildings using the mouse lymphoma assay (MLA ). Although a thin-layer chromatography analysis of extracts isolated from test buildings showed the presence of well known mycotoxins (ochratoxin A and sterigmatocystin), the test extracts as well as control extracts did not reveal mutagenic potential after 3-hr exposure without or with metabolic activation as tested with MLA . Negative results were also obtained after 24-hr treatment with every test sample in the absence of S9 fraction. The data indicates that extracts prepared from control and test buildings under experimental conditions did not induce mutations affecting the expression of the thymidine kinase gene in the cultured L5178Y TK^+/- cells.