After labelling of human and ovine prolactine by the lactoperoxydase method, each prolactine shows at least three molecular forms : amorphous aggregates of large molecular weight, one dimere (M.W. no. 50 000) and one monomere (M.W. no. 25 000). The monomere and the dimere can spontaneously give rise to the aggregates. But the dimere never give monomere and monomere never give dimere. Urea 8 M alone have not dissociating action on the polymeres or on the dimere. But urea 8 M plus mercaptoethanol transforms polymeres into dimeres and monomeres.