The peripheral lymphocytes of rabbits may be stimulated in vitro to undergo blast transformation by the F(ab′)2 and F ab fragments of sheep globulin containing antibody activity to rabbit IgG. This indicates that the secondary biologic properties of immunoglobulin molecules and the capacity to precipitate with antigen are not necessary for blastogenic activity. The initial event in the induction of blast transformation by specific antiserum and by specific antigen may be the same mechanism, the primary union of antigen and antibody.