Low concentrations of antibodies, specific to human respiratory syncytial virus (RSV) have been shown to enhance virus replication in human monocytic cell lines by several workers. In the present study, replication of bovine RSV in ovine peripheral blood monocytes was shown to be enhanced in the presence of low concentration of bovine RSV-specific antibodies. Antibodies had no enhancing effect on virus replication in secondary lamb testis cells or monocytic cell lines derived from peripheral blood monocytes. The possible effects of low titres of bovine RSV-specific antibodies on the development of clinical disease were examined by inoculating groups of lambs with a mixture of virus and antibodies and assessing the severity of clinical disease and by measuring venous oxygen (PO2) and carbon dioxide (PCO2) tensions, as hypoxia has been associated with respiratory diseases. Inoculation of bovine RSV and virus-specific antibody complexes to lambs did not enhance clinical disease and had no effect on the clinical chemistry, haematology and PO2 and PCO2 tensions. Groups of lambs inoculated with virus alone or virus-antibody complexes developed significant humoral and cellular immune responses. There was no significant difference in the cellular immune responses of lambs exposed to virus alone and lambs exposed to virus-antibody mixture, as measured by virus-specific lymphocyte transformation or by cytotoxicity assays but the period of virus shedding was longer in lambs inoculated with a mixture of virus and immune serum. (C) 1998 Elsevier Science B.V.