Investigations of natural history trade-offs between reproduction and immunity are common throughout the literature. Most previous studies of such trade-offs have focused on how resources can be drawn from immune response to fuel reproduction. Our results demonstrate that resources also can be shifted from reproduction to immunity. Immunologically-challenged male northern cricket frogs (Acris crepitans) expressed reduced investment in reproduction. Spermatic cyst diameter, germinal epithelium depth, and gonadosomatic index were smaller in antigen-injected males relative to those injected with a sham (saline injected) and noninjected control animals. Although body size increased in all groups during this study, linear growth and body mass did not appear to be significantly different among these three treatment groups. These results demonstrate indirectly that in A. crepitans immune response may increase metabolic demand for resources and fuel that need from the stores normally used to support male reproduction. We speculate that anything eliciting an immune response in this species may reduce male fertility, so pathogens and toxins at levels that are currently believed to be relatively harmless may impact populations in ways we could not previously predict.