Rabbit lymphocytes may be stimulated in vitro with specific antiallotype sera to transform into "blast" cells and to synthesize DNA. This transformation only occurs when the donor cells are obtained from a rabbit having a given gamma-globulin allotype (As4) and these cells are cultured in the presence of an antiserum prepared against the given allotype (As4). Heterologous (sheep, goat, and guinea pig) anti-rabbit gamma-globulin sera also induce significant blast transformation and DNA synthesis in rabbit lymphocytes. Allotypic transformation and DNA synthesis are due to 7S antiallotype antibodies and do not require complement. The degree of transformation and rate of DNA synthesis is related to the concentration of antibody. Incubation of the appropriate cells with the antiallotype antibody for as short a time as 15 minutes results in a significant degree of "blast" transformation, indicating that the recognition of the antiallotype specificity in the cells and stimulation of the cellular changes leading to eventual transformation is rapid. The activity of the antiallotype sera as measured by transforming or haemagglutinating capacity, may be absorbed by lymphocytes of the appropriate allotype, but is not absorbed by lymphocytes from a donor rabbit not having the allotype to which the antiserum is directed. Transformation does not occur with mixtures of lymphocytes from different rabbits even if 1 donor is immunized against an allotype present in the other donor. Peripheral rabbit lymphocytes can also be induced to undergo "blast transformation" in vitro by phytohaemagglutinin and staphylococcal filtrate. The lack of demonstrable leucoagglutinins in staphylococcal filtrate and antiallotype serum indicates that agglutination is not a necessary prerequisite to the induction of blast transformation.