Summary A new method is described for the enumeration of single cells producing antibody against a pure protein preparation from Salmonella adelaide flagella. Cells with adherent bacteria, cultured in nutrient agar, give rise to bacteria adherence colonies which can be counted under low power magnification. The method discloses an extremely low level of background of 0.03 adherence colonies/106 spleen cells. Critical tests have affirmed that adherence colonies are due to cells actually taking part in the immune response. Since the method is based on an antigen known to be highly immunogenic and capable of evoking secondary responses as well as tolerance under defined circumstances, all major phenomena of immunity can be studied at the cellular level.