Abstract: A method for early detection of mastitis in cows, effected during normal milking of cows in a milking parlor, by monitoring the milk taken from each cow for the presence of milk clots, comprises cutting the vacuum milk line to each milking station and inserting a filter-detector therein between the claw and the recorder jar. Each filter-detector comprises a flat-sided transparent casing with a lateral slot which receives a self-sealing slide carrying the filter element. The slide carrier preferably has an integral bypass aperture so that vacuum conditions at the claw end are retained even after the filter element has become blocked. The slide carrier is preferably constructed so that, when in position in the casing, a seepage passage remains between the slide and the casing inner wall, so that cleansing fluid can pass during normal in-place cleaning of the installation.