An African (Kenyan) strain of Babesia bigemina, Muguga (B(2-1)), was inoculated into a calf from a stabilate and blood from the calf was used to establish the parasite in vitro. The strain has been cultured continuously for 20 months, initially in bovine erythrocytes with 60% adult bovine serum, later, with 50%. Cultures were incubated at 37 degrees C in RPMI 1640 medium with a gas mixture of 1% O2, 5% CO2, 94% N2. Adaptation in vitro was demonstrated when serum from a calf which had recovered from infection with B(2-1) bound to proteins of Mr 46 kDa, 49 kDa, 52 kDa, 61 kDa and 72 kDa on Western blots of B(2-1) antigens from cattle blood but did not recognise the 49 kDa or 52 kDa antigens from in-vitro-derived parasites. These proteins were considered specific for B(2-1), as they were not recognised by the same serum on profiles of a Mexican isolate of B. bigemina or an African isolate of B. bovis (Kwanyange). After 9 months of in vitro culture, a stabilate of the cultured parasite was injected into two splenectomised calves and one intact calf. The calves experienced a drop in packed cell volume and low parasitaemias but recovered spontaneously. Two of these animals, one splenectomised and one intact, were challenged with virulent B(2-1) and experienced only mild babesiosis, in contrast to a previously uninfected calf also challenged with B(2-1), which had to be euthanised after 5 days with severe babesiosis.