Experimental infection of Bali cattle (Bos sondaicus) with blood from a case of Jembrana disease caused a well-defined pathological response. The disease was generalized except for the central nervous system which did not show significant cellular reactive changes. Following a general lymphoreticular reaction in the first week (Phase 1), the predominant effect was an intense non-follicular proliferative response by reticulum (dendritic) cells and lymphoblastoid cells in lymphoid organs (Phase 2) and a similar infiltrative and proliferative process in liver, kidneys, adrenal medulla and elsewhere. In the kidneys, glomerular hypercellular swelling occurred which was associated with uraemia. Pulmonary alveolar cells reacted strongly to infection by swelling and proliferation especially in the anterior lobes. Mononuclear cell infiltration accompanied this response. In lymph nodes and spleen, a marked lymphoid follicular reaction and plasma cell formation developed from the fifth week of infection (Phase 3). The cytology and histological distribution of proliferative changes in the lymphoid system suggests that, during the acute phase of the disease, a predominantly T lymphocytic reaction takes place which may be associated with transient immunosuppression. Residual lesions occurred up to day 60 post-infection. In all affected tissues pleomorphic basophilic, intracytoplasmic inclusions occurred, especially from the second week until about the fifth week of infection, but persisted in small numbers for more than 2 months. By light microscopy both minute basophilic granular forms (Type 1) and large intravascuolar (Type 2) inclusions were consistently found in reticular cells, lymphoblasts, macrophages including Kupffer cells, pulmonary alveolar cells and occasionally in vascular endothelium. These structures appear to be of diagnostic value.