The enzymatic activity of monocytes was studied with light microscopic semi-quantitive analysis and microspectrophotometer, including B-glucuronidase, acid phosphatase, N-acetyl-?-glucosaminidase, peroxidase and ?-naphthyl acetate esterase in peripheral blood smears prepared by hcparinazed micropipet centrifugal method in some normal donors and patients. The data of normal groups have been obtained. The results tested by microspectrophotometry have indicated that the larger the area of monocyte was, the stronger the five kinds of enzymatic activity in monocyte was. In comparison with the normal groups, there were significant alterations of the different kinds of enzymatic activity of monocytes in the patients with cir- rhosis, systemic lupus erythematosus, pemphigus, untreated malignant tumor and radiotheraped tumor respectively. The significance of the results described above have been discussed in the paper.