Human respiratory syncytial virus and bovine respiratory syncytial virus (BRSV) suppress lymphocyte responses to mitogens. In the present study, the possible effects of BRSV on some functions of antigen-presenting cells (APC) were investigated by exposing ovine monocytic cells to the virus before their use as APC. The depletion of monocytes from peripheral blood mononuclear cells resulted in the near total abrogation of proliferative responses to phytohaemagglutinin (PHA). Reactivity was restored by the addition of homologous monocytic cells derived from ovine peripheral blood monocytes as APC. The exposure of these monocytic cells to BRSV for 48 h before their use as APC significantly reduced the proliferative responses of uninfected ovine lymphocytes to PHA. Furthermore, the exposure of bovine peripheral blood monocytes and bovine and ovine monocytic cell lines to BRSV for 48 h reduced their capacity to phagocytize latex beads. (C) 1998 W.B. Saunders Company Limited.