The effects of mild Sarcocystis miescheriana infection on blood enzymes and body weight were compared in stress-sensitive (halothane positive) and in stress-insensitive (halothane negative) pigs. Ten halothane-positive pigs (Group A) and ten halothane-negative pigs (Group C) with a mean body weight of 36 kg were each inoculated orally with 50,000 sporocysts of S. miescheriana. Twelve halothane-positive pigs (Group B) and ten halothane-negative pigs (Group D) served as non-infected controls. Five days before infection (a.i.) and 58 days post infection (p.i.) all pigs were myostress challenged (creatine kinase test). During the whole period of the experiment (3 weeks a.i. to 13 weeks p.i.) body weights were determined and blood samples taken at weekly intervals. Creatine kinase (CK), aspartate aminotransferase (ASAT) and Sarcocystis antibody titres were determined. The infection induced slightly elevated temperatures (max. 42.8 degrees C) and transient reduced food intake during the 2nd and 3rd week p.i. The CK values of the infected pigs in Groups A and C increased from Day 28 p.i. onwards, and were significantly higher than those of the pigs of the non-infected controls (Days 35-77 p.i.). The ASAT values of the infected groups (A and C) increased from Day 21 p.i. onwards, and were significantly higher than those of the non-infected controls from Days 28-77 p.i. The myostress injection at 5 days a.i. (1st CK test) resulted in significantly higher CK and ASAT values in stress-sensitive pigs.(ABSTRACT TRUNCATED AT 250 WORDS)