The presence of normal bacterial flora in the intestinal tract is thought to protect against colonization by pathogens. Only a few specific examples of this protection have been demonstrated for bacterial pathogens and protozoan infections. Mice from one commercial breeding farm were found to be less susceptible to infection with Giardia lamblia than were isogenic mice from another facility. When mice were housed together, resistance to infection was readily transferred to normally susceptible mice. After resistant mice were treated with neomycin, differences in susceptibility to infection were shown to be due to differences in the resident flora present in these mice. These results suggest the possible use of probiotic therapy for prevention of G. lamblia infections and may help explain some of the variability of outcomes seen in G. lamblia infections in humans.