More than 50% of human peripheral lymphocytes in culture will transform to a blastlike cell when incubated with phytohemagglutinin. To a lesser degree, the lymphocytes of a sensitive individual incubated with the appropriate antigen will also undergo blastogenesis. This phenomenon occurs not only in antigen-antibody systems associated with circulating antibody (e.g., tetanus) but also in those systems associated with delayed sensitivity (e.g., tuberculin). This study demonstrates that lymphocytes from a sensitive individual will also undergo blastogenesis when incubated with the appropriate antigen from the reagin group, in this case, ragweed antigen.