The immunomodulatory role of Mycobacterium phlei against intracellular blood protozoan Babesia bigemina was demonstrated following experimental immunisation and challenge in bovine calves. A lysate of erythrocytes infected (6 x 10(9)) with B. bigemina was used as a source of dead antigen either with Freund's complete adjuvant (FCA) or with a trypsinised culture of M. phlei as a non-specific immunomodulation (NSI) agent with appropriate controls. Following virulent challenge with B. bigemina infected erythrocytes (1 x 10(7)), the NSI printed calves showed 100% protection, while the dead antigen alone with FCA afforded 75% protection. The protective status of the immunising regimes was studied by clinicopathological parameters and assessment of humoral and cell-mediated immune responses. The role of babesial dead antigen and the effects of M. phlei on the development of immunity to B. bigemina is discussed.