The effect of the divalent and univalent fragments of the immune globulins of burro anti-rabbit lymphocyte serum (BALS), sheep anti-rabbit IgG (SAG) and rabbit anti-rabbit allotypic IgG (RAG) on rabbit lymphocytes in vitro was tested. The divalent (F(ab′)2) fragments of each immune globulin stimulated lymphocytes to transform. The univalent fragments (Fab) of SAG and RAG were also stimulatory, but the univalent fragment of BALS was not stimulatory. A different type of reaction site distribution or stimulatory mechanism for BALS in contrast to SAG and RAG is suggested.