Excretion in milk fat of Aroclor 1254 (a mixture of polychlorinated biphenyl) by mature beef cows (Hereford and Hereford cross) was studied. Three groups of 6 cows each were fed primarily a corn silage diet characterized by 1) clean silage (stored in an uncontaminated silo), 2) silage stored in a silo coated with a sealant containing Aroclor 1254, and 3) clean silage to which 200 mg Aroclor 1254 per head daily was added (approximately 2 to 3 mg/kg body weight per day). Treatments were started approximately 3 mo prior to parturition and extended 1 mo after parturition, at which time treatments were discontinued, and cows and nursing calves were placed on pasture. Milk was sampled in the period between parturition and 132 days after discontinuance of treatments. Fat from the milk of cows fed silage from the silo which had not been sealed with the Aroclor 1254 product contained .69 to 1.59 ppm Aroclor 1254 throughout the 164-day lactation. Fat from cows fed silage from the silo treated with the Aroclor 1254 sealant contained more than ten times as much Aroclor 1254 (15.7 to 18.4 ppm) for 32 days as compared with the controls and then dropped to one-half that figure through the 164th day. Fat from the milk of cows fed 200 mg Aroclor 1254 per day contained from 119 to 150 ppm through the first 32 days and then dropped to 39 to 51 ppm through day 164.