The teratogenicity of cobalt chloride (CoCl2) was tested by the FETAX (Frog Embryo Teratogenesis Assay: Xenopus) procedure in the South African frog, Xenopus laevis. In five assays, beginning at 5 h post-fertilization, groups of Xenopus embryos were incubated for 96 h in media that contained CoCl2 at concentrations ranging from 1.8 x 10(-6) to 1.8 x 10(-2) mol/L; control groups were incubated in the same medium without added CoCl2. At 101 h post-fertilization, surviving embryos were counted, fixed in formalin, and examined by microscopy to score malformations and measure head-to-tail lengths. In control embryos, survival was greater than or equal to 95% and malformations were less than or equal to 5%. Malformations were found in greater than 99% of embryos exposed to Co2+ levels greater than or equal to 56 mumol/L. Co2+)-exposed embryos showed a concentration-related pattern of malformations, comprising gut malrotation, ocular anomalies, kinked tail, craniofacial dysplasia, cardiac deformities, and dermal blisters. Other concentration-dependent abnormalities, not categorized as malformations, included stunted growth, edema, ventral distention, and hypopigmentation. The median embryolethal concentration (LC50) of CoCl2 was 10.4 (SE +/- 0.4) mmol/L; the median teratogenic concentration (EC50) was 25 (SE +/- 2) mumol/L; the teratogenic index (TI = LC50/EC50) was 416 (SE +/- 13), indicating that CoCl2 is a potent teratogen for Xenopus laevis.