Eight- and nine-week-old Hungarian Landrace pigs were tested with halothane as described by Laky et al. (1985). Immediately after the test blood samples were taken for determination of the activity of serum creatine kinase (CK), creatine kinase MB (CK-MB) isoenzyme, aldolase (ALD), lactate dehydrogenase (LDH) and alpha-hydroxybutyrate dehydrogenase (alpha-HBDH). Elevated creatine kinase, creatine kinase MB isoenzyme and aldolase activities indicating enhanced susceptibility to stressors were found in 92% of the halothane reacted and 16% of the halothane non-reacted animals. In these individuals the activities of lactate dehydrogenase and alpha-hydroxybutyrate dehydrogenase were also high. Data of the literature show a close relationship between enhanced susceptibility to stressors and halothane reaction in pigs. It was suggested, therefore, that determination of the activity of appropriate serum enzymes might be used for detecting this enhanced susceptibility.