Sterile sections of rubber and stainless steel pipe were inserted sequentially into a milking installation and soiled with fresh raw milk over a period of 5 d. The resultant adherent microbial population was removed and the generic composition of mesophilic and psychrotrophic types was determined. In all cases Acinetobacter spp. were found to predominate (59.5–75.6%). The generic composition of the raw milk used to soil the milking unit (with inserted pipe section) was determined once during each 5-d soiling period. In general the milk was found to contain a mixed flora in which Gram-positive organisms predominated.