An attempt was made to sterilize a newly erected building of approximately 1200 m.3 (43,000 ft.3), specially designed for the breeding of specific-pathogen-free mice, rats and guinea-pigs. Two methods of treatment were used, namely an ampholytic surface acting biocide/formaldehyde aerosol followed after 2 days by formaldehyde vapour. Bacteriological examination was made of 100 sites in the animal rooms, staff quarters and general service area before and after both treatments. Identification of the bacteria isolated was based upon their morphological appearance on laboratory media incubated aerobically and their reaction to Gram's stain. Organisms were isolated from 72/100 sites before treatment, from 50 sites after the first treatment, and from 13 sites after the second treatment. The bacteria that survived both treatments were of several species.