Thiabendazole (TBZ) was tested for teratogenicity using Jcl:ICR mice. TBZ suspended in olive oil was given orally to pregnant mice at different stages of organogenesis. All foetuses were removed from the uterus on day 18 of gestation, and were examined for external and skeletal anomalies. In mice given 700, 1300 or 2400 mg TBZ/kg body weight/day on days 7-15 of gestation, dose-dependent external and skeletal anomalies, especially cleft palate and fusion of vertebrae, were observed. In mice given a single dose of TBZ (2400 mg/kg) on any one of days 6-13, an increased number of malformations was observed. Various types of malformation occurred, especially in the mice treated on day 9. Reduction deformity of limbs was found in mice given TBZ on days 9-12, a change that has not previously been observed to occur spontaneously in normal ICR mice in our laboratory. In order to determine a dose-response relationship, groups of mice were given one of 17 doses of TBZ (30-2400 mg/kg) on day 9 of gestation. The number of litters having foetuses with reduction deformity of limbs and of those having foetuses with skeletal fusion increased in proportion to the dose of TBZ. The regression lines of Y (probit response) on X (log dose) for reduction deformity of limbs and for skeletal fusion were Y = 2.47X - 3.65 and Y = 1.54X + 0.48, respectively. The effective doses (ED1) for the two malformations were 362.0 and 26.4 mg/kg, respectively.