OBJECTIVE To study the effect of platelet lysate (PL) on chondrogenic differentiation of human umbilical cord derived mesenchymal stem cells (hUCMSCs) in vitro. METHODS Umbilical cords were voluntarily donated by healthy mothers. The hUCMSCs were isolated by collagenase digestion and cultured in vitro. The surface markers of the cells were detected by flow cytometer. According to different components of inductive medium, the cultured hUCMSCs were divided into 3 groups: group A [H-DMEM medium, 10% fetal bovine serum (FBS), and 10%PL]; group B [H-DMEM medium, 10%FBS, 10 ng/mL transforming growth factor beta1 (TGF-beta1), 1 x 10(-7) mol/L dexamethasone, 50 microg/mL Vitamin C, and 1% insulin-transferrin-selenium (ITS)]; and group C (H-DMEM medium, 10%FBS, 10 ng/mL TGF-beta1, 1 x 10(-7) mol/L dexamethasone, 50 microg/mL vitamin C, 1%ITS, and 10%PL). The hUCMSCs were induced in the mediums for 2 weeks. Toluidine blue staining was used to detect the secretion of chondrocyte matrix. Immunofluorescence method was used to identify the existence of collagen type II. The expressions of Aggrecan and collagen type II were detected by semiquantitative RT-PCR. RESULTS Flow cytometer results showed that the hUCMSCs did not express the surface markers of hematopoietic cell CD34, CD45, and human leukocyte antigen DR, but expressed the surface markers of adhesion molecule and mesenchymal stem cells CD44, CD105, and CD146. Toluidine blue staining and immunofluorescence showed positive results in group C, weak positive results in group B, and negative results in group A. Semiquantitative RT-PCR showed the expressions of Aggrecan and collagen type II at mRNA level in groups B and C, but no expression in group A. The mRNA expressions of Aggrecan and collagen type II were higher in group C than in group B (P < 0.05). CONCLUSION Only 10%PL can not induce differentiation of hUCMSCs into chondrocytes, but it can be a supplement to the induced mediums. PL can improve hUCMSCs differentiating into chondrocytes obviously in vitro. This study provides new available conditions for constructing tissue engineered cartilage.