The hyperplastic reaction of the regional lymph node in response to the first and second set skin homograftings has been studied. The magnitude of the reaction of the lymph node was expressed quantitatively as variations in weight, DNA and RNA contents. Skin homografting was performed on the dorsum of the left ear of adult rabbits and the lymph node most proximal to the base of the ear was examined. The regional lymph node of the contralateral side served as controls. The chief results obtained are as follows:
1) In the autografting performed as a control, there is little response of the regional lymph node.
2) In the first set homografting, the regional lymph node reveals a strong proliferative response, and the weight, DNA and RNA contents increase rapidly after homografting reaching to the peak on the 7th day, and thereafter decrease gradually to be normal on the 21st day.
3) In the second set homografting, the peak of proliferative response of lymph node is seen earlier on the 5th day, and the response is markedly accentuated.
4) In both the first and the second set responses, the peak of the proliferative response of the lymph node is seen at nearly the same time of the onset of rejection of the homografted skin.
5) In the first set reaponse, thc lymph node of the contralateral side shows a slight proliferative response on the 5th day, whereas such a response is not observable in the second set homografting.
6) Tne histological examination of the lymph node shows that the proliferation of plasma cells is relativly slight in both the first and the second set responses.