Animal-infective forms of a West African Trypanosoma vivax stock were grown in culture for three months using Minimum Essential Medium (MEM) with Earle's salts, supplemented with 20% inactivated goat serum over fibroblast-like cell lines isolated from the embryo of Microtus montanus or of an East African Galla crossbred goat at 36.5 degrees C and in 4% CO2 - 96% air. The bloodstream trypanosomes used to initiate the culture had been isolated from an infected goat. The cultured organisms grown in this system could be subcultured, were infective for mammalian hosts, retained their morphological characteristics and virulence, and could be readily established in Glossina morsitans centralis from goats injected with the cultured T. vivax.