Serum IL-4 and gamma interferon levels after transplantation of lethally irradiated BALB/c mice with 5 × 10⁶ bone marrow and 4 × 10⁵ spleen cells from naive (▴) or tolerant (▵) CBA mice. Three recipient mice from each treatment group were sacrificed every other day and equal volumes of serum samples were pooled for the assays. The sera were assayed in duplicate. Variations between duplicates were less than 17%. Similar results were obtained when the experiment was repeated.