Bovine respiratory syncytial virus (BRSV) depressed the proliferative reactivity of normal ovine peripheral blood lymphocytes to phytohaemagglutinin (PHA). This BRSV-induced reduction in proliferative reactivity was not reversed or ameliorated by the addition of (1) indomethacin or flunixin meglumine, substances known to inhibit the production of prostaglandins, or (2) the cytokines, interleukin-1 (IL-1) and interleukin-2 (IL-2), or (3) rat growth factor. The results suggest that the suppression of ovine lymphocyte reactivity to PHA associated with BRSV was not caused by the release of cyclooxygenase products such as prostaglandins, or the production of inhibitors of IL-1 or IL-2. (C) 1998 W.B. Saunders Company Limited.