Faecal egg counts, haematocrits, erythrocyte potassium contents and serum iron concentrations were determined in 1005, 3- to 5-month-old Merino lambs infected with a single dose of 11 000 Haemonchus contortus larvae. Live-weight gain and wool growth also were recorded. Lambs were infected in six different groups over a 3-year period. When infections were terminated after 5 weeks, faecal egg counts in the six infected groups had reached a peak of 5170 to 20 339 eggs per g (average 12 909), haematocrits had declined to between 196 and 309 ml/1 (average 233), erythrocyte potassium contents had risen to between 16·7 and 37·5 mequiv. per 1 (average 31·5) and serum iron concentrations, in some cases following an erratic course, had dropped to between 0·512 and 1·546 mg/1 (average 0·946).

Of the three haematological parameters, haematocrit correlated best with faecal egg count (r = 0·7 in four of six infected groups). However, in two groups with low faecal egg counts this correlation was much lower (r = 0·3). Erythrocyte potassium concentration and serum iron concentration significantly correlated with variability of haematocrit not accounted for by faecal egg count, suggesting that both erythropoiesis and iron availability influence the degree of anaemia.

The effect of H. contortus infection on productivity of lambs was best predicted by haematocrits: for each further 0·01 proportional decrease in haematocrit, a 0·03 reduction of live-weight gain over a 9-week post-infection period, a 0·007 reduction in clean wool growth and a 0·004 reduction in fibre diameter over a 4- to 9-week period were observed. Some evidence was obtained indicating a tolerance level of anaemia at approximately 280 ml/1 packed cell volume.