Using tritiated thymidine and an autoradiographic technique, the small lymphocytes in the inoculum were labelled and found to migrate in large numbers into the cortex of the lymph nodes and into the Peyer's patches of the host animal, and in smaller numbers into the white pulp of the spleen. Within 24 hours isotope, previously present in the DNA of small lymphocytes, appeared in a number of the large pyroninophilic cells which were a characteristic feature of the spleen and lymph nodes in this early phase of runt disease.

When the large lymphocytes in the inoculum were labelled they were found to migrate to the red pulp of the spleen, medulla of the lymph nodes, and the Peyer's patches and the lamina propria of the small intestine. Later some labelled small lymphocytes appeared at these sites.

These findings suggest that:

(1) Some small lymphocytes are immunologically competent cells, and

(2) After introduction into the circulation of a newborn rat, these same small lymphocytes are the first cells to react with the antigens of the host, and in the process they become transformed into large pyroninophilic cells capable of division. The large lymphocytes seem to play little part in this initiating of an immunological response, but do give rise to some small lymphocytes.