Background:　To　investigate　the　oxidized　low　density　lipoprotein　(oxLDL)　on　U937　cell　ATP-binding　cassette　transporter　A1　(ABCA1)　mRNA　expression　and　cholesterol　efflux　situation.　Methods:　Human　U937　cells　were　incubated　with　gradient　concentrations　of　oxLDL(0,　25,　50,　75,　100,　125mg/L),　and　then　dyed　by　oil　red　O　to　estimate　the　content　of　intracelluar　lipid　and　detect　the　expressing　quantity　of　ABCA1　mRNA　by　Real-time　Fluorescence　quantitative　PCR　simultaneously.　Calculating　the　cholesterol　efflux　rates　by　using　the　scintillation　counter　to　detect　the　amount　of　H3　-cholesterol　in　each　well　cell　culture　plate　and　medium.　Results:　Real-time　Fluorescence　quantitative　PCR　analysis　showed　that　the　expression　levels　of　ABCA1　mRNA　in　monocytes　were　lower　than　basal　line　when　not　intervened　with　oxLDL,　and　increased　drastically　with　oxLDL　stimulation,　significant　difference　compared　with　controls(P＜0.01),　and　reached　the　highest　level　at　oxLDL　50mg/L,　nevertheless,　continously　increasing　the　concentration　of　oxLDL　above　50mg/L,　the　expression　decreased.　So　is　the　outflowing　rate　of　intracelluar　lipid.　Oil　red　O　dyeing　results　also　suggested　that　celluar　lipid　content　was　the　highest　when　intervened　with　125mg/L　oxLDL,　and　increased　most　obviously　at　50mg/L　oxLDL.　Cholesterol　outflow　result　also　demonstrated　that　cholesterol　outflow　rate　related　with　the　ABCA1　mRNA　expressing　quantity.　Conclusion:　With　the　increase　of　intervening　concentration　of　oxLDL　on　U937cells,　the　exprssion　of　ABCAl　mRNA　represented　that　rising　befor　50mg/L　oxLDL,　and　then　decreasing,　reaching　the　top　point　at　50mg/L　oxLDL.　so　was　the　change　in　the　outflowing　rate　of　intracelluar　lipid.　©　2012　IEEE.