Fibrocytes (Fb) are a population of circulating leukocytes reported to be capable of presenting antigen to CD4(+) T lymphocytes. In contrast, no information is available about their capacity to stimulate CD8(+) cytolytic T lymphocyte (CTL) responses. To this end, Fb were isolated from porcine blood to investigate their ability to stimulate CTL responses using a classical swine fever virus model. The isolated Fb (referred to as primary Fb) displayed the phenotype previously reported for mouse and human Fb, particularly in terms of the surface proteins necessary for antigen presentation, major histocompatibility complex (MHC) classes I and II, and CD80/86. These primary Fb endocytosed and degraded antigen efficiently. In absence of exogenous stimuli, endocytosis and MHC II expression were lost when the Fb were passaged and cultured. Treatment of such secondary Fb with interferon-gamma (IFN-gamma) restored the MHC II expression. The primary and secondary Fb were capable of stimulating antigen-specific CD4(+) T lymphocytes relating to previous reports. In addition, an efficient stimulation of virus-specific CD8(+)CTL was measured in terms of CD8(+) T cell proliferation, IFN-gamma production, and cytotoxic activity. This was noted even at low Fb/T lymphocyte ratios, at which dendritic cells were less efficient. Although IFN-gamma pretreatment of Fb was not necessary for this function, it could enhance the Fb activity. These results demonstrate that Fb are efficient, accessory cells for the presentation of viral antigen to specific CD8(+) CTL.