 water samples from several ponds in minnesota were evaluated for their capacity to induce malformations in embryos of xenopus laevis . 
 the fetax assay was used to assess the occurrence of malformations following a 96-hr period of exposure to water samples . 
 these studies were conducted following reports of high incidences of malformation in natural populations of frogs in minnesota wetlands . 
 the purpose of these studies was to determine if a biologically active agent(s ) was present in the waters and could be detected using the fetax assay . 
 water samples from ponds with high incidences of frog malformations ( affected sites ) , along with water samples from ponds with unaffected frog populations ( reference sites ) , were studied . 
 initial experiments clearly showed that water from affected sites induced mortality and malformation in xenopus embryos , while water from reference sites had little or no effect . 
 induction of malformation was dose dependent and highly reproducible , both with stored samples and with samples taken at different times throughout the summer . 
 the biological activity of the samples was reduced or eliminated when samples were passed through activated carbon . 
 limited evidence from these samples indicates that the causal factor(s ) is not an infectious organism nor are ion concentrations or metals responsible for the effects observed . 
 results do indicate that the water matrix has a significant effect on the severity of toxicity . 
 based on the fetax results and the occurrence of frog malformations observed in the field , these studies suggest that water in the affected sites contains one or more unknown agents that induce developmental abnormalities in xenopus . 
 these same factors may contribute to the increased incidence of malformation in native species.imagesfigure 1figure 2figure 3figure 4figure 5figure 6figure 7figure 8 
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