Contaminants, including pesticides, can affect an organism's health by weakening its immune system, potentially making it more susceptible to parasites. We used one measure of immune function, leukocyte profiles, and compared these for 82 Northern Leopard Frogs, Lithobates ( = Rana) pipiens, occupying five wetlands with different exposures to the herbicides atrazine and metolachlor, and tested for associations between blood parasites and leukocyte profiles. Hepatozoon spp. (likely H. clamitae) were the only blood parasites detected and were found in only seven (8.5%) frogs. These parasites were associated with fewer eosinophils per leukocyte and higher heterophil∶lymphocyte ratios, although we cannot distinguish cause and effect. Heterophils, lymphocytes, and eosinophils per total number of leukocytes also differed significantly among wetlands, but variation could not be attributed consistently to pesticide contamination.