1. The antigenicity of homologous cortical and cancellous bone has been investigated in eighty-four rabbits.

2. The primary immune responses which occur in lymph nodes draining homografts of fresh tissues (Burwell and Gowland 1961, 1962) have been used as a histological indicator of the antigenicity of fresh homologous cortical bone freed from soft tissues.

3. The secondary immune responses which occur in lymph nodes draining homografts of fresh marrow-containing iliac bone (Burwell 1962a, b) have been used also as a histological indicator of the antigenicity of homografts of 1) fresh cortical bone freed from soft tissues, 2) fresh marrow-free iliac bone, and 3) mairow-containing iliac bone treated by boiling, freezi ng, freeze-drying and merthiolate solution.

4. It is found that whereas fresh homologous cortical bone fails usually to produce cytological evidence of a primary response in the regional lymph nodes, fresh homologous cortical bone chips inserted into the drainage areas of lymph nodes sensitised previously to donor ..tissue evoke constantly cytological evidence of a secondary response.

5. Fresh homologous marrow-free iliac bone inserted into the drainage areas of lymph nodes sensitised previously to donor tissue does not produce detectable evidence of a secondary response.

6. Homografts of boiled marrow-containing iliac bone do not elicit a secondary response in lymph nodes previously sensitised to donor tissue.

7. Previous work has shown that homografts of frozen (–20 degrees Centigrade) marrow-containing iliac bone do not evoke a primary response in lymph nodes draining such grafts. In the present work it is shown that similar frozen homografts inserted into the drainage areas of lymph nodes previously sensitised to donor tissue evoked a secondary response in three of six lymph nodes.

8. Homografts offreeze-dried marrow-containing iliac bone fail usually to evoke a secondary response in lymph nodes sensitised to donor tissue.

9. Homografts of marrow-containing iliac bone treated by immersion in merthiolate solution before being inserted into the drainage areas of lymph nodes previously sensitised to tissue from the donor elicited a secondary response in three of five lymph nodes.

10. Knowledge concerning the antigenicity offresh and treated homologous bone is discussed in the light of recent work.