An early proliferative response follows the mixing of homologous spleen or lymph node suspensions obtained from two unrelated rabbits. The rate of incorporation of radioactive thymidine has been used as a quantitative measure of this response. Thymus cells do not respond to homologous cell suspensions but may on occasion serve to stimulate the response in homologous spleen or lymph node cells. Homologous erythrocytes or autologous tissues do not stimulate a response.

No response occurs if the two cell populations are separated by a Millipore membrane.

Autoradiographic studies have established that 1 to 2 per cent of the intial cell population is involved in the response and they are large undifferentiated cells by the time they can first be identified as responders. There was no morphological evidence of any cellular interaction and the viability of mixed suspensions was not measurably different from that observed in separate suspension.

Simultaneous additive responses could be obtained to homologous cells and to antigen when cell suspensions from immunized rabbits were used. The interaction of the cell populations from the two rabbits did not appear to suppress the response of each to antigen.

The speed and magnitude of the response were in everyway comparable with the secondary response of cell suspensions from immunized rabbits exposed to the immunizing antigen. No evidence was obtained of any enhancement of the response to homologous cells by prior immunization with homologous tissues, but the possibility that it had occurred was not rigidly excluded.

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