Glossina morsitans were infected with two cloned stocks of T. congolense. The proboscides, foreguts and midguts of infected flies were then used as sources of trypanosomes in vitro at 28 degrees C in the presence of bovine dermal collagen explants. Cultures were established in which trypanosomes differentiated into adhering colonies of epimastigote forms which could then be maintained and subcultured in Eagle's Minimum Essential Medium supplemented with foetal calf serum for over 40 weeks. Within 2-4 weeks of establishment of each culture or subculture the epimastigote trypanosomes differentiated into metacyclic trypanosomes which could be harvested from supernatant medium at concentrations of 1 X 10(5)-3 X 10(6) parasites/ml. These organisms were used to induce the formation of local skin reactions in rabbits. Successful cultivation of infective trypanosomes appeared to depend on the initial adhesion of the parasites to the surface of the flask where they subsequently differentiated first into epimastigote and then to metacyclic forms.