Lymphocyte subpopulations entering the liver and surrounding the rejection sites during a 9-day period after infection of immune sheep with Taenia hydatigena were identified with the aid of monoclonal antibodies against lymphocyte cell surface markers. Viable lymphocytes were isolated from the liver tissue and stained by indirect immunofluorescence for subsequent flow cytometry analysis. Over the first 6 days after challenge infection a marked increase in the ratio of T-helper to T-suppressor/cytotoxic lymphocytes was observed. SBU-T19+ lymphocytes, a CD5+ T-cell subpopulation uniquely identified in the sheep, were present in small numbers in sheep liver both before and after infection. There was a large, continuous increase of sIg+ B-cells over the 9-day observation period after infection. Eosinophils were the predominant granulocytes in the liver of infected sheep. The exact location of the leucocyte subpopulations in respect to the rejection sites in infected liver was determined by in-situ immunoperoxidase staining of frozen liver sections. The evolution of the parasite-induced leucocyte response was characterized by the appearance of a central core of eosinophils surrounded by increasing numbers of CD4+ helper T-cells. CD8+ (suppressor/cytotoxic) and SBU-T19+ T-lymphocytes were present in much smaller numbers and by day 9 after infection were located predominantly around the periphery of the lesions. Distinct foci of tightly packed B-cells developed within the lesions and increased dramatically in size over the 9-day observation period. At this time, lesions appeared as compact aggregations of leucocytes encircled by a second band of eosinophils. Both T- and B-lymphocytes within the lesions stained positive for class II major histocompatibility antigens.