Objective Matrix metalloproteinase-12 (MMP12) also known as macrophage elastase, is capable ofdecomposing almost all extracellular matrix components and is required for monocyte recruitment. MMP12 has been found tobe involved in cancer invasion and metastasis. In addition, MMP12 is also closely related to adipose tissue. This studyinvestigated whether MMP12 regulates the macrophages in blood and white fat in MMP12 knockout (MMP12^{- / -}) mice.Methods MMP12^{- / -} mice were raised and genotyped, and littermate MMP12^{+ / +}male mice were used as controls. Theroutine blood indexes of the two groups of mice were analyzed, and the macrophage markers were detected by flowcytometry. Pathdogy using HE staining and immunohistochemistry was performed to determine the macrophages in adiposetissue after MMP12 deletion. Results (1) The genotype of MMP12^{- / -}mice and expansion was identified. (2) Comparedwith those of littermate MMP12^{+ / +}male mice, parameters including RBC count, HGB level, PLT count, and monocytecount and percentage were significantly reduced in MMP12^{- / -} male mice. However, the proportions of lymphocytes andeosinophils were significantly increased compared with those of control mice, but their absolute counts did not differsignificantly. ( 3) Further result suggested that the proportion of macrophage-specific marker CD11b and F4 / 80 doublepositive cells was significantly decreased in the blood, and their absolute count was also reduced. (4) The pathdogy usingHE staining and immunohistochemistry confirmed that macrophage (CD68+) expression was significantly increased in thewhite adipose tissue of MMP12^{- / -}male mice. Conclusions These results suggest that macrophage changes may be relatedto MMP12 knockout. The significance is that MMP12 may regulate the development of macrophages, the macrophages arereduced in the blood and increased in white fat.