Two cleaning-in-place disinfectant Systems (alkaline–acid, alkaline–CI2) were tested on wild bacteria isolated from a milking line. Most of the isolated strains were Gram-negative and capsule-positive. The effect of milk and cream on the resistance of the wild strains to the cleaning Systems was tested. Suspension tests did not show the resistance of bacteria whereas the surface test showed high resistance towards all cleaning-agents. Milk had a protective effect only on the surfaces whereas cream showed mild protection in suspension and complete protection on the surfaces. It was concluded that the growth pattern of contaminants on surfaces needs further attention and the laboratory suspension tests do not correlate with the practical situation. Disinfectant tests based on bacterial growth on the surface are of importance.