Cyanobacteria are prevalent in the freshwater environment, reaching critical mass in harmful algal blooms. These organisms produce a variety of toxins including endotoxins such as lipopolysaccharides (LPS), which have been previously shown to decrease glutathione-S-transferase (GST) activity in zebrafish (Danio rerio) embryos. GST plays a vital role in detoxification response during oxidative stress and provides a first line of defense after toxic heavy metal insult, before increased metallothionein expression. Although some attention has focused on cyanobacterial LPS, little research has focused on effects of concurrent exposures with other toxicants. Because cyanobacterial LPS can alter detoxification enzymes including GST, we hypothesized that cyanobacterial LPS could potentiate metal toxicity. This study investigated the effects of LPS from two cyanobacterial species, Lyngbya spp. and Microcystis aeruginosa, on cadmium toxicity in zebrafish embryos. Forty-eight-hour CdCl 2 LC 50 values showed that coexposure of cadmium and Lyngbya LPS or Microcystis LPS resulted in significantly increased cadmium toxicity in comparison with cadmium alone. However, increased cadmium toxicity was not due to decreased GST activity as initially hypothesized. In concurrent Microcystis LPS-cadmium exposures, GST activity was significantly increased in comparison with control embryos at all time points and cadmium concentrations sampled. Concurrent Lyngbya LPS-cadmium exposures also resulted in increased GST activity at most exposure concentrations. These results indicate that regardless of mechanism, cyanobacterial LPS can potentiate the toxic effects of heavy metals. This represents a significant risk for aquatic organisms exposed to combinations of LPS and metals in the environment.