Specific antisera directed against all six of the well characterised allotypic determinants of rabbit IgG (As1, 2, 3, 4, 5, and 6) are capable of inducing blast transformation and DNA synthesis when added to lymphocyte cultures obtained from donor rabbits having the appropriate IgG allotype. Mixtures of antisera directed against two different allotypic determinants induce a "summation" of transformation and DNA synthesis over and above the effect of mixtures of two antisera directed against the same allotypic determinant. This summation effect is observed regardless of whether the antisera which have been mixed are directed against allotypic determinants controlled by the same locus or by different loci. The finding that summation occurs with mixtures of two antisera directed against both the allotypic determinants of a double homozygote rabbit (As1, 6) suggests that lymphocytes from the peripheral blood may be primed to produce only one or the other of the two polypeptide chains of IgG, but not both.