Grazing sheep are invariably subjected to natural nematode parasitic challenge. Anthelmintic resistance is becoming widespread among parasites and alternative control measures are urgently needed. A potential alternative control method is breeding for improved resistance (Woolaston and Windon, 2001). The aim of this study is to investigate the genetic control of the component traits of parasite infection (worm length, worm burden and worm fecundity) and indicator traits of infection, and quantify genetic relationships between these traits. Indicator traits investigated in this study are fructosamine concentration, immunoglobulin A (IgA) activity, eosinophil counts and pepsinogen concentration. Fructosamine concentration reflects average glucose and protein concentrations and rates of protein turnover. IgA is a secreted antibody and is of key importance in gut infections. Eosinophils are a type of white blood cell associated with parasitic infection. Pepsinogen is a digestive enzyme precursor, which increases concentration in the plasma following the breakdown of intercellular junctions in the abomasum.