The invention relates to a pH-sensitive polymer which is a (meth)acrylate copolymer composed of 20 to 65% by weight acrylic and/or methacrylic acid units and 80 to 35% by weight units of C.sub.1- to C.sub.18-alkyl esters of (meth)acrylic acid, characterized in that it has a molecular weight in the range from 1 000 to 50 000 g/mol, and brings about at least 60% haemolysis at pH 5.5, and less than 5% haemolysis at pH 7.4, in a concentration of 150 .mu.g/ml in a cytotoxicity test with human red blood cells. The invention further relates to the use of the pH-sensitive polymer as carrier for pharmaceutically effective biomolecules or active pharmaceutical ingredients and as ingredient of corresponding dosage forms.