Objective To explore the effects of low density lipoprotein(LDL) containing immune complexes(IC) on the mRNA expression of matrix metalloproteinase-1(MMP-1) and matrix metalloproteinase-1(TIMP-1) in U937 cells.Methods U937 cells were incubated with native LDL,oxidized LDL or LDL-IC,respectively.mRNA expression of matrix metalloproteinase-1(MMP-1) and tissue inhibitor of metalloproteinase-1(TIMP-1) in U937 cells were measured by reverse transcriptase polymerase chain reaction(PT-PCR).Results Normal U937 cells hardly expressed MMP-1,but highly expressed TIMP-1.N-LDL had no effect on the expression of MMP-1 and TIMP-1 mRNA.Ox-LDL can induce slightly expression of MMP-1 and markedly down-regulate expression of TIMP-1.Similarly,nature and oxidized LDL-IC had significantly more effect on the mRNA expressions of MMP-1 and TIMP-1 than that of control(P<(0.001)).Conclusion LDL-IC stimulated the expression of MMP-1 mRNA and down-regulated the expression of TIMP-1 mRNA in U937 cells.