The effect of low-level exposure to formaldehyde on oral, nasal, and lymphoycte biological markers was studied prospectively in a group of 29 mortician students who were about to take a course in embalming. During the 85-day study period, the subjects performed an average of 6.9 embalmings and had average cumulative formaldehyde exposures of 14.8 ppm-h, with an average air concentration of 1.4 ppm during embalming. Since the average time spent embalming was 125 min, formaldehyde exposures calculated as an 8-h time-weighted average were 0.33 ppm on days when embalmings were done, which was less than the Occupational Safety and Health Administration permissible exposure limit of 0.75 ppm. Epithelial cells from the buccal area of the mouth showed a 12-fold increase in micronucleus frequency during the study period, from 0.046 +/- 0.17/1000 cells preexposure to 0.60 +/- 1.27/1000 cells at the end of the course (P < 0.05). Nasal epithelial micronuclei increased 22%, from 0.41 +/- 0.52/1000 cells to 0.50 +/- 0.67/1000 cells (P = 0.26). In blood cells, the frequency of micronucleated lymphocytes increased 28%, from 4.95 +/- 1.72/1000 cells to 6.36 +/- 2.03/1000 cells (P < 0.05), while sister chromatid exchanges decreased 7.5% (P < 0.05). A dose-response relationship was observed between cumulative exposure to formaldehyde and increases in buccal micronuclei in the 22 male subjects but not in the 7 female subjects. We conclude that low-level exposure to formaldehyde is associated with cytogenetic changes in epithelial cells of the mouth and in blood lymphocytes. These cytogenetic effects may be useful as markers of biologically effective dose.