This study was designed to examine the immunological privilege of the hamster cheek pouch. A comparison of the cheek pouch with the non-immunologically privileged flank was carried out by investigating responses to Glaxo BCG at the tissue, cellular and humoral levels using Histological methods, Mantoux Tests, Lymphocyte Transformation Tests, Passive Haemagglutination Tests and Immunoelectrophoresis Tests. These studies showed no differences in the responses to BCG in the cheek pouch or flank but do not preclude the possibility that the responses elicited in the cheek pouch differed qualitatively or quantitatively from those in the flank. In response to a single injection of BCG into the cheek pouch or flank compact and non-caseating epithelioid cell granulomas developed locally, with no evidence of the infection extending into the surrounding tissue or progressing to peripheral sites in the body. These findings could be related to the low virulence of Glaxo BCG for the hamster. Animals that had received a single injection of BCG or a rechallenge injection of BCG in Incomplete Freund's Adjuvant (IFA), were Mantoux negative and may also be related to the low virulence of Glaxo BCG for the hamster. The levels of specific and non-specific lymphopro-liferation to BCG and PHA, respectively were biologically insignificant in animals that had received a single injection of BCG and in those that had received a rechallenge injection of BCG in IFA. These findings can probably be attributed to the conditions of culture in the Lymphocyte Transformation Test. No specific agglutinating antibody was detected in the sera of animals that had received a single injection of BCG or a rechallenge injection of BCG in IFA. These findings could be related to the unreliability or lack of sensitivity of the Passive Haemagglutination Test.