Virus-specific T cell responses were measured in cattle seropositive for bovine viral diarrhoea virus (BVDV) and compared with those from BVDV-seronegative animals. CD4+ and CD8+ T cells were purified and co-cultured in vitro with autologous, BVDV-infected monocytes over a time-course to assess the kinetics of the proliferative response. Supernatants from parallel T cell cultures were harvested and the presence of the cytokines interleukin (IL)-2, IL-4 and interferon-gamma (IFN-gamma) were measured by ELISA (IFN-gamma) or by a bioassay (IL-2 and IL-4). CD4+ and CD8+ T cells from all seropositive, but not seronegative, cattle proliferated specifically in response to BVDV-infected monocytes. Measurement of cytokines in the supernatants from proliferating T cell cultures showed that the CD4+ T cell response was type 2-like, with extremely high levels of B cell growth factor and IL-4 activity together with comparatively low levels of IL-2 activity and IFN-gamma protein. The CD8+ T cell response, although more variable, appeared to be type 1-like, with increased IL-2 and IFN-gamma but no IL-4 or B cell stimulatory activity.