We propose a simple assay to assess the importance of seven vertebrate species as food sources for Mepria spinolai, a wild arthropod vector of Trypanosoma cruzi (the agent of Chagas' disease). Rabbits were immunized with serum proteins from one of each of the seven species. After titration, a consensus 1/100,000 dilution of the immune sera detected vertebrate serum proteins in the intestinal contents of 48.9% of 131 insects tested. The high proportion of negative samples is consistent with previous information indicating that these insects can withstand prolonged fasting periods. Alternatively, they may have fed on a different animal species than those used to produce the antisera. In about 70% of the positive samples, only one species of serum protein was detected. All pre-immune sera were negative. In 67% of the positive vectors, rabbit immunoglobulins were detected directly by means of a specific goat antibody. Thus, rabbits may play a role in T. cruzi transmission.