Antiallotypic stimulation of rabbit peripheral lymphocytes in vitro may be completely inhibited by the addition up to 36 to 42 hr after lymphocyte cultures have been initiated of serum containing the allotypic specificity to which the stimulating antiserum is directed. This inhibition results in a reversal of the transformation process so that enlarged lymphocytes return to a smaller size without going through mitosis. Piggy-back augmentation is also inhibited by the addition of antigen reacting with the primary stimulating antiserum. The observation that the continued presence or contact of stimulating antiserum with reacting cells is required to drive the cells to completion of the transformation process has important implications in regard to activities of lymphoid cells that require a proliferation phase for expression.