Two groups of lambs raised free of sporozoan infection were inoculated with Sarcocystis tenella sporocysts and compared with controls. Lambs from Group 1 were inoculated with 5000 sporocysts and those in Group 2 were given 20,000. Transient increases in rectal temperatures occurred between 23 and 39 days post-inoculation (dpi), although the lambs appeared normal and retained their appetites. Packed cell volumes (PCV) of lambs given 20,000 sporocysts decreased dramatically from 28 to 38 dpi after which they slowly returned to near pre-inoculation levels by 99 dpi. The anaemia was normocytic/normochromic. White cell counts (WCC) rose in infected lambs from 49 dpi, reflecting principally an increase in lymphocyte numbers. Plasma albumin of Group 2 decreased at 28 dpi and remained depressed until the experiment was terminated at 99 dpi. Plasma globulin of infected groups increased from 31 (Group 2) and 35 dpi (Group 1). Plasma alkaline phosphatase (ALP) of Group 2 decreased from 28 dpi and remained depressed to 99 dpi. Lactate dehydrogenase (LDH) of Group 2 was elevated at 24 and 28 dpi and from 42 to 78 dpi, while aspartate aminotransferase (AST) of the same group was elevated from 45 to 66 dpi. Creatine kinase (CK) of Group 2 was elevated from 52 to 71 dpi.