Sheep were sensitized by weekly infections with Teladorsagia circumcincta over a 9-week period. After a 12-week rest, sheep were divided into four groups and killed without challenge or 3, 5 and 10 days post challenge (DPC) with 50000 L3. Recovery of challenge larvae from abomasal scrapings was highest at 3 DPC while no parasites were recovered by 10 DPC. Abomasal lymph nodes (ALN) of challenged sheep were significantly larger at 5 DPC, coinciding with an increase in the proportion of CD4 T cells and a decrease in CD21+ cells, probably reflecting the loss of CD21 from terminally differentiated antibody secreting cells. A significant increase was observed in gammadelta-TCR+ cells at 3 DPC in the ALN, while their number slightly decreased in the abomasal tissues throughout the challenge period. The number of tissue eosinophils was dramatically increased after challenge compared with the unchallenged controls, with a peak at 3 DPC, coinciding with the peak in larval recovery. CD4+ cells significantly increased in the abomasal tissues at 5 DPC, while no changes in globule leucocytes were observed until 10 DPC. Antibody-secreting cell probes (ASC-probes) generated from the ALN showed highest reactivity against larval antigens at 5 DPC. This reactivity was predominantly directed against regions between 90 and 100 kDa and 30-35 kDa in the L3 preparation and lower molecular weight antigens in the L4. No reactivity was observed against the adult extract. The 30-35 kDa antigen seemed to exist as a high molecular weight complex in L3 homogenate and was not susceptible to protease K treatment, suggesting it may be non-protein in nature.