Reduced host-reactive Th frequencies in spleens of IL-12–protected mice. Lethally irradiated B6 mice received A/J BMC and spleen cells plus TCD B6 BMC with or without IL-12 on day 0. B6-reactive and A/J-reactive (specificity control) Th frequencies per spleen were determined by limiting dilution analysis on days 4 and 7 after BMT. The total numbers of B6 (host)-reactive (A) and A/J (donor)-reactive (B) Th per spleen, as determined by the product of the Th frequency and the total number of spleen cells obtained from each individual animal, are shown. (⧫) GVHD control mice; (•) IL-12–protected mice; (*) normal A/J mice; (□) normal B6 mice. Results of three experiments are combined.