Ovine isotype-specific antibody responses to the parasitic larval stages of the abomasal nematode Ostertagia circumcincta were measured in a simple, indirect enzyme-linked immunosorbent assay. Analysis of variance of replicate tests showed that the assay was very reliable. There was substantial variation among individual sheep in their IgA and IgG1 responses even though the sheep had been matched for breed, age and sex, were born on the same farm, were reared identically and had the same history of exposure and challenge with O. circumcincta. The local IgA responses to a somatic extract of fourth-stage larvae were very similar to responses to excretory-secretory products of fourth-stage larvae. The responses to third stage larvae were correlated with the responses to fourth stage larvae. There was a negative correlation between parasite-specific plasma IgG1 and parasite-specific plasma IgA responses. There was only a moderate association between IgA responses in the mucus and the plasma. Therefore, antibody responses measured in plasma cannot be easily extrapolated to antibody responses in the abomasal mucus.