A cultured microflora obtained from the caecum of a "normal" mouse was given to 4 groups of germfree mice and was supplied 1x, 2x, 3x and 4x respectively at 5-day intervals. Another group received a 10(-7) dilution of the caecal flora while a group associated with an 'SPF' flora served as control. The difference (measured by 8 parameters) between mice supplied with the cultured flora or with a 10(-7) dilution, both given once only, was small. Supplying the flora 3x resulted in more 'normal' mice compared with mice which received the flora once or twice. The caeca of specified-pathogen-free mice contained more bacteria per gram (microscopic bacterial count), less aerobic and anaerobic bacteria per gram (viable counts), while the yield as percentage of the microscopic bacterial count was lower as compared with the group to which a cultured flora was supplied 4 times.