The present invention relates to mixed esters of hyaluronic acid, wherein the hydroxyl groups are partially esterified with retinoic and butyric acids. These mixed esters are characterized by specific degrees of esterification and by a high ratio between the degree of substitution with butyric acid and retinoic acid. They exhibit a high anti-proliferative activity associated with activation of cell differentiation, with consequent clinical relevance in the treatment of hyper-proliferative pathologies and in particular of solid and systemic tumors.