Rabbit peripheral blood lymphocytes that have been stimulated in vitro to transform into blast cells by antiimmunoglobulin allotypic sera and then blocked by addition of serum containing the allotype with which the stimulating antiserum reacts may be restimulated by subsequent removal of the blocking serum and readdition of the stimulating antiserum. The extent of stimulation produced by stimulation-block-restimulation is the same as that produced by the continuous presence of the antiallotypic serum (continuous stimulation). However, if unstimulated cultures are incubated in vitro in neutral serum before stimulation with antiallotypic serum (delayed stimulation), the extent of stimulation is less than that produced by continuous stimulation or by stimulation-block-restimulation. Prestimulation of double homozygous lymphocytes with antiserum to one allotypic specificity activates the cells to respond to delayed restimulation with antiserum to the other allotypic specificity (mixed allotypic sequential stimulation, MASS). This MASS activation is not observed in cultures of mixtures of lymphocytes with antiallotypic sera to specificities contributed by different donors. These results indicate that continued contact of the transforming cells with the stimulating agent (signal maintenance) is required to consummate the transformation process and that cultures that are stimulated and blocked may be restimulated by a second specific signal.