Responses to Taenia taeniaeformis infection were studied in mice with severe combined immunodeficiency (scid), which lack functional T and B lymphocytes. In the early phase of infection, accumulation of polymorphonuclear leucocytes (PML) occurred around the larvae in the liver of scid mice and their immunocompetent counterparts, C.B-17, (a BALB/c strain, genetically resistant to this parasite). PML accumulation continued until encapsulation of developing larvae by fibroblasts (14 days p.i.), and subsequent fibrosis resulted in granuloma formation. No infiltration of eosinophils or macrophages around larvae was observed in scid mice prior to granuloma formation, while in C.B-17 mice infiltration was observed as early as 5 days p.i., when specific antibodies could not be detected in the circulation. Most larvae were destroyed by 14 days p.i. in C.B-17 mice. In scid mice the larvae survived but the host capsules (cysts) were thin and most contained blood at 42 days p.i. In these cysts, inflammatory cells were observed on the larval surface and in invaded parasite tissue. Hepatocyte coagulation necrosis adjacent to larvae was commonly found in C.B-17 mice by 5 days p.i., while it did not occur in scid mice throughout these experiments. These results suggest that in host responses to larval T. taeniaeformis, PML accumulation and encapsulation by fibrosis are T and B cell independent, while eosinophil and macrophage infiltration, as well as resistance to infection, are T and/or B cell dependent. Additionally, there may be an association between host cell necrosis around larvae and T and/or B cell responses.