Sister-chromatic exchanges measured in the peripheral lymphocytes of 15 non-smoking medical students after exposure to formaldehyde during a 24-week anatomy class showed a small but significant (p=0.0468) increase when compared with samples obtained from the same individuals immediately before exposure. Mean frequencies of sister-chromatic exchange of cultured peripheral lymphocytes were 5.40+/-0.24 from the samples before exposure and 5.87+/-0.22 from the same samples after exposure. Breathing-zone air samples collected by formaldehyde monitoring kit with digital colorimeter (SKC) showed a mean concentration of 0.72+/-0.02 ppm formaldehyde.