Graft-vs.-host (GVH) reactivity of parental lymph node (LN) cells was assayed by measurements of 3H-thymidine incorporation in vivo in spleens of irradiated F1 recipients. Preincubation of parental LN cells with vesicular stomatitis virus (VSV) for 2 h at 37 degrees C followed by washing resulted in an 85-90% reduction in splenic radioactivity, as did injection of VSV on days 0-2 after recipients received untreated parental LN cells. In contrast, 3H-thymidine incorporation in the spleens or irradiated F1 hosts was not affected by VSV when F1 bone marrow cells were incubated with the virus. In addition, preincubation of F1 B cells with VSV still allowed these syngeneic B cells to be recruited into proliferation by mitomycin-treated parental LN cells. The inhibitory effect of VSV, thus, seems to be specific for T-cell proliferation. These observations suggest that viral immunosuppression might be capable of being developed into a useful strategy for selective deletion of lymphocytes capable of reacting against histocompatibility antigens and initiating GVH reactions.