Haematological and biochemical changes were studied in 10 Friesian calves after vaccination with attenuated Theileria annulate (T. annulata) vaccine and post challenge with the virulent strain. The parameters were compared with those of 21 animals, naturally infected with annulata. The vaccinated gp. showed no appreciable changes either post vaccination or post challenge. On the other hand, the naturally infected animals and the non vaccinated gp. post challenge revealed a significant fall in Hb, PCV and total erythrocyte count. Levels of total serum protein, albumin, calcium, phosphorus, magnesium, sodium and potassium decreased and those of globulins, GPT, GOT and bilirubin increased with no changes in kidney functions. Several of these biochemical alterations have been explained in relation to the injury to the liver and other organs.