A method of preparing an isolated serum fraction of platelet rich fibrin, cell cultures comprising said serum fraction and its use as a cell culture additive. The invention also relates to increasing proliferation rate of chondrocytes, to the treatment of articular or joint diseases and to increasing the proliferation rate of mesenchymal stem cells. The isolated serum fraction of platelet rich fibrin (PRF), is prepared by providing platelet rich plasma (PRP) without the addition of an anticoagulant; clotting the PRP to obtain a coagel of PRF; and separating the coagel to isolate the serum fraction which comprises an activated platelet releasate; and further provides for the isolated serum fraction obtained by such method, and its medical use.