The occurrence of cells which produce 'natural" bactericidal and haemolytic antibodies (the so-called background plaques) was studied by the plaque method in lymphatic tissues of germ-free and conventionally reared rabbits of various ages. In conventional rabbits the cells which produce "natural" bactericidal antibodies against E. coli 086 appear early after birth in organized intestinal lymphatic tissue, their number increases during development, then they appear in mesenteric lymph nodes and spleen and their number decreases due to aging. Natural haemolytic antibodies are produced throughout the life predominantly in the spleen of conventional rabbits. In germ-free rabbits the appearance of "natural" bactericidal or haemolytic antibodies was not demonstrated during the tested period of 16 weeks of life. Also when comparing the immunological capacity after parenteral immunization or after stimulation of cells from various organs cultivated in diffusion chambers, the response in germ-free rabbits was very low or lacking. The differences between the reactivity of germ-free and conventional rabbits were especially marked when studying the specific and non-specific (polyclonal) response after administration of E. coli suspension. The nature and possible causes underlying the differences in immunological reactivity in various species of germ-free animals are discussed.