The upregulation of endothelial cell MCP-1 production by ox-LDL is a major initiating event in atherogenesis. HDL and PON1 retard the oxidation of LDL and therefore may retard endothelial cell MCP-1 production. The endothelial cell line EAhy926 was incubated with ox-LDL in the presence and absence of HDL and PON1 and the production of MCP-1 was measured by ELISA. Human HDL and PON1 significantly inhibited the in vitro oxidation of LDL and completely prevented the ox-LDL induced increase in MCP-1 production by endothelial cells. Ostrich HDL that does not contain PON1 was unable to prevent LDL-oxidation or the production of MCP-1 by endothelial cells. PON1 attenuates the ox-LDL induced MCP-1 production by endothelial cells. This is one, early, mechanism by which PON1 may be anti-atherogenic. (C) 2004 Elsevier Inc. All rights reserved.