In the active induction of the suppressive effect on the transferred cells it was found that neonatal rabbits reacted as vigorously as 1 kg rabbits to the prior injection of a given number of rabbit leucocytes pooled from prospective donors of lymph node cells. The suppressive effect was dose-related, and within the range of number of leucocytes used was similar for both age groups of recipients. An attempt to detect a difference in the response during the first few days after leucocyte injection, before the full suppressive effect is reached, failed to show any difference between rabbits of the two age groups.

Since it had been found possible to transfer the suppressive effect passively with sera obtained from older rabbits injected with rabbit leucocytes, attempts were made to do so with sera obtained from neonatal rabbits injected with similar numbers of pooled adult rabbit leucocytes. No consistent suppression of transferred lymph node cells was observed with sera from neonatal rabbits, even with relatively large amounts of such serum. In sera of rabbits which had been injected with rabbit leucocytes at the age of 4 to 6 weeks, suppressive antibody could be detected. When anti-rabbit-leucocyte serum obtained in adult rabbits was injected into neonatal and 1 kg recipients at a given volume per gram of animal weight the suppressive effect of the serum was of similar extent in the two groups of recipients.

In the adoptive transfer of the lymph node cell suppressive effect, by cells of lymph nodes draining the sites of injection of pooled rabbit leucocytes, it was found that the popliteal lymph node cells of neonatal rabbits were as effective as those of 1 kg rabbits. Splenic cells of neonatal rabbits were also effective, when an adequate number of rabbit leucocytes had been injected intravenously.

Thus, in conferring adoptive immunologic response, as in active immunization, the neonatal rabbits were as effective as the older rabbits in their response to homotransplantation antigens, in contrast to the considerable difference in concentration of the suppressive antibody in sera of neonatal and older rabbits injected with rabbit leucocytes.