Formaldehyde is the chemical substance illegally used for food preservation in meat, vegetables and fruit. A study on the antiproliferative effect and cytogenetic effect of formaldehyde on human lymphocyte was undertaken. Heparinized blood from 30 volunteers was collected and treated with formaldehyde concentrations of 0.036, 0.072, 0.15, 0.3, 0.576, 0.8 and 1.152 mg/mL, respectively, for 24 hours. Viable lymphocyte count by hemocytometer and MTT assay were carried out for detecting the antiproliferative effect of formaldehyde on human lymphocyte. Lymphocyte culture and G-banding technique were carried out for detecting the cytogenetic effect of formaldehyde. The results showed that the numbers of viable lymphocyte in the control group were 3.45 x 10(4) cells/mL. The numbers of viable lymphocyte in the experimental subgroups were 3.03 x 10(4), 2.69 x 10(4), 2.36 x 10(4), 2.17 x 10(4), 1.92 x 10(4), 1.68 x 10(4) and 1.04 x 10(4) cells/mL, respectively, at 24 hours. The value of IC(50) was 0.92 mg/mL. The formaldehyde concentrations of 0.036, 0.072, 0.15, 0.3, 0.576, 0.8 and 1.152 mg/mL effect the lymphocyte antiproliferation (p < 0.05). Loss of chromosome was the cytogenetic effect by the formaldehyde concentration of 0.036 and 0.072 mg/mL in this study. It is concluded that formaldehyde has the antiproliferative effect and cytogenetic effect on human lymphocyte.