We previously reported the emerging role of CD137–CD137L interaction in inflammation and atherosclerosis. The mechanism of CD137–CD137L interaction may be related to a variety of signaling pathways. The most important signaling pathway involves the activation of phospholipase C(PLC) which induces the diacylglycerol–protein kinase C(DAG–PKC) and the inositol trisphosphate-intracellular free calcium (IP 3 -[Ca 2+ ]i) pathway. In the current study, we investigated whether CD137–CD137L interaction can stimulate the PLC signaling pathway in human umbilical vein endothelial cells (HUVEC). The diacylglycerol (DAG) and inositol trisphosphate (IP 3 ) levels in HUVEC were measured by radioenzymatic assay. The activity of protein kinase (PKC) was detected by its ability to transfer phosphate from [γ- 32 P]ATP to lysine-rich histone. The [Ca 2+ ]i concentrations were measured by flow cytometric analysis. The DAG level and PKC activity were increased in a concentration-dependent, biphasic manner in HUVEC induced by anti-CD137. PKC activity was mainly in the cytosol at rest, and then translocated to the membrane when stimulated by anti-CD137. Similarly, rapid IP 3 formation induced by anti-CD137 coincided with the peak of the DAG level. Moreover, anti-CD137 induced peak [Ca 2+ ]i responses including the rapid transient phase and the sustained phase. However, anti-CD137L suppressed the activation of the DAG–PKC and IP 3 -[Ca 2+ ]i signaling pathway, which was stimulated by anti-CD137 in HUVEC. In conclusion, the data suggested that CD137–CD137L interaction induces robust activation of the PLC signaling pathway in HUVEC.