A milking system includes a milking device, a milk line for carrying milk, and a sampling unit arranged to take a sample from the milk line and test the sample for the presence or concentration of at least one substance. The sampling unit is provided with a housing. The housing includes a supply reel with a tape that carries reagent, and a sample supplier. The sampling unit includes a tape displacer displacing the tape, a sample analyser, and a used tape collector device collecting tape with reagent to which a sample has been supplied. The supply reel at a surface thereof includes a desiccant, or is completely made of a desiccant material. This provides such a large volume of desiccant that the correspondingly high moisture absorption capacity ensures an uninterrupted useful life of the sampling unit, and thus of the milking system.