Reports of declining and deformed amphibians have been increasing in the last few years and this issue has become an important environmental problem of the last decade. This problem involves various infectious pathogens, including chytrid fungi and parasitic flatworms (trematodes). A possible explanation for increased incidence of amphibian declines is that something in the environment is affecting amphibian immune systems. Atrazine is the most widely used herbicide in North America and has been detected as a common environmental pollutant. The purpose of this research was to test the idea that low concentrations of atrazine may have an effect on the immune systems of amphibians, rendering them more susceptible to pathogen infection by using hemolytic plaque assays and hemocytometer counts. Rana pipiens showed a statistically significant difference in the number of plaques produced after 1 month of atrazine treatment but did not show statistically significant differences in treatments after 2 months of atrazine exposure. These results suggest that the immune system of frogs may be negatively affected by at least short term atrazine exposure.