SUMMARYSeventy-four recipients of related donor renal allografts were tested for the presence of cellular immunity to specific donor lymphocyte antigens using the direct migration inhibition factor (MIF) assay. Responses on the assay fell into one of the following three statistically distinct groups: (1) greater than 20% inhibition of macrophage migration, (2) nonresponsiveness, 10% of control migration, and (3) greater than 12% stimulation of macrophage migration. Migration stimulation was shown to be reproducible and to correlate well with a very benign post-transplant clinical course. The production of migration stimulatory factor appears to be an immunological response analagous to the production of migration inhibition factor.