Intravenous injection of mice with BCG cell wall vaccine induces granuloma in the lung. The granuloma induced as measured by lung weight showed marked strain differences; C3H/HeMs bred in this Institute (C3H), C3HeB/FeJ, P/J and A/He were low responders, whereas C57BL/6 and AKR/He were high responders. When C57BL/6 and C3H were compared, the responsiveness appeared to be inherent in each strain of mice providing strong evidence for genetic control in this phenomenon. Breeding experiments suggested that multiple loci, including a MHC-linked locus, were involved although the role of the MHC-linked loci was marginal.