A four-month-old calf had a clinical history of pyrexia, anaemia, weight loss and behavioural abnormality. Clinical examination revealed evidence of regenerative anaemia and a lymphocytosis which was characterised by a relatively large B cell population. The calf deteriorated clinically while under observation and its prescapular and prefemoral lymph nodes became enlarged. Examination of a blood smear revealed the presence of a large number of circulating Trypanasoma theileri. Serological examination showed the presence of the invariant, stage-specific, trypanosome surface antigen, ISG70 and antibodies against ISG70. ISG70 was first identified in the bloodstream forms of Trypanosoma brucei and has not previously been found in T theileri. Clinical recovery was associated with an increase in packed cell volume, a decrease in the levels of circulating anti-ISG70 antibodies and the complete disappearance of circulating ISG70.