Infection with Trypanosoma cruzi was established in the reduviid vector, Dipetalogaster maximus, by repeated feedings on mice with high parasitaemias, and metacyclic trypomastigotes (IMT) were collected in insect urine after blood meals. The infectivity of IMT in mice was assessed by placing varying numbers of organisms, ranging from 5 to 5000, on to the conjunctivae or oral mucosa of anaesthetized animals. Half of the mice exposed to as few as 20 IMT by either route became parasitized, and the minimum inoculum size that resulted in all mice becoming infected was 640 IMT by the ocular route and 1250 IMT in the mice given parasites by mouth. Mice were immunized by tail vein infusion of irradiated IMT. Animals in the immunized group and in two control groups were then challenged by deposition of IMT on to the oral mucosa. Two of five immunized mice and nine of 10 comparison animals developed acute T. cruzi infection after challenge. These results indicate that IMT produced in this system are highly infective and that inocula containing 500-1000 IMT applied to the conjunctivae or oral mucosa constitute a gentle contaminative challenge. Moreover, our findings suggest that sterile protection against a contaminative challenge may be inducible by immunization with IMT, but experiments involving larger numbers of animals must be performed to resolve this question.