The in vitro activity of antidog antilymphocyte serum in three different tests was correlated with in vivo immunosuppressive activity as measured by skin allograft survival. The results indicated the following. (1) Lymphocytotoxic titre often correlated well, but outstanding exceptions reduced the predictive value of this test. (2) In vivo opsonization correlated well, but the reproducibility and sensitivity of this test were poor. (3) Rosette inhibition correlated accurately with immunosuppressive activity, and reproducibility and sensitivity were excellent.