A number of haematological parameters were monitored in eight-month-old Merino wethers, six infected with 10,000 Haemonchus contortus and 13 controls. Blood loss caused by infection was reflected in a decrease in packed cell volume (r = -0.78; P less than 0.001), and was correlated with faecal egg excretion (r = 0.89; P less than 0.001). Infected animals showed a decline of serum iron content and increased serum transferrin levels. Erythrocyte potassium concentration showed a high correlation with erythropoiesis as measured by iron59 clearance from blood (r = 0.79; P less than 0.001). As potassium assays are quick, inexpensive and samples can be stored for long periods, this parameter seems particularly useful in large experiments.