A number of plasma biochemical parameters were examined in five outbreaks of runting in broiler chickens. In four of the five outbreaks, runts showed consistent elevations in plasma amylase activity and reductions in glutathione peroxidase activity. In two of the five outbreaks the plasma vitamin E concentration was reduced, as was the activity of plasma alkaline phosphatase. A highly significant number of runted chickens were found to have pancreatic degeneration, elevated plasma amylase activity and reduced plasma glutathione peroxidase activity, compared with non-runted chickens. The implications of these changes are discussed in relation to the aetiology of runting and stunting syndrome and, in particular, the possible involvement of selenium.