Guinea pigs were injected in the footpads with either purified diphtheria toxoid or recrystallized egg albumin in Freund adjuvant without mycobacteria. Each guinea pig was then skin-tested only once with the specific antigen and bled for antibody determination. After injection of the sensitizing antigen, a latent period occurred during which neither sensitivity nor circulating antibody could be detected. A period of delayed sensitivity followed wherein circulating antibody could not be discerned and which could be transferred by lymph node cells. Ultimately, the Arthus type sensitivity developed, accompanied by circulating antibody. The duration and severity of reactions to homologous antigens during the last 2 phases varied with the antigen and with the dose. An increase in the sensitizing dose decreased the duration of the delayed type of allergy, a decrease in the dose prolonged the delayed type. Inclusion of mycobacterium in the sensitizing inoculum tended to introduce delayed sensitivity earlier and delay the onset of Arthus type sensitivity. When specific precipitate in antibody excess was included with the toxoid in the sensitizing dose, the onset of the Arthus phase was hastened. When lymph nodes from a large number of sensitized donors were removed during the latter part of the latent period, recipients of the cells showed a delayed type sensitivity.