Regardless of official recommendations, the inappropriate use of homemade hair creams has became a popular practice in Brazil and high formaldehyde content in the 'progressive straightening' creams has been reported. In the present work, three of these creams were analyzed by spectrophotometric, chromatographic and genotoxic assays in order to evaluate mutagenic risks associated with the uncontrolled addition of formaldehyde at contents higher than those allowed by regulation. The ultraviolet and Fourier-transformed infrared absorption spectra showed characteristic signals that can be assigned to formaldehyde, although with different relative intensities, revealing distinct compositions. Using high-performance liquid chromatography 1.6-10.5% w/v formaldehyde was quantified. Antibacterial activity was detected in all creams. At 0.10 microg per plate, one of them showed positive mutagenicity induction (P < 0.05) in the Salmonella/microsome assay using the TA100 strain. The measurement of beta-galactosidase induction in the SOS chromotest by this cream, at dosages of 10-100 microg per assay, was positive (P < 0.05) in Escherichia coli PQ37 and OG100 strains. Our data show a more intense genotoxic response than those reported before for formaldehyde, suggesting that this compound may be acting synergistically with any unknown components in the creams or perhaps these unspecified components by themselves might have significant genotoxic potential. We call attention to the popular use of homemade formulations of cosmetics, such as hair straightening creams, because they can contain mutagens that could increase the incidence of neoplasia in those people who use them.