The immunological response of captive naturally-infected Rocky Mountain bighorn sheep to the lungworm, Protostrongylus stilesi, was examined throughout the seasonal cycle of parasite activity using a cell adherence assay for circulating antibody. The net effect of all stable serum factors on cell adherence was examined by exposure of normal cells to larvae in the presence of test serum (decomplemented-serum test). Homocytotropic antibody was examined by exposure of washed normal cells to larvae after passive sensitization with serum from infected animals (washed-cell test). Although the washed-cell test did not show a clear association with parasite activity, the net ability of sera to promote adherence showed a significant increase during periods of elevated Protostrongylus activity. Fecal counts of Muellerius, Nematorirus and coccidia were not associated with the results of either assay. Significant differences among animals were detected only for the washed-cell test when the effects of parasitism were considered.