A cultured human monocytic leukemia cell line, J-III, was used to make heterologous antiserum against human peripheral monocyte surface antigens. To eliminate non-specific antibodies, this antiserum was extensively absorbed with human erythrocytes, neutrophils, nonglass adherent mononuclear cells (mainly lymphocytes) and fetal kidney homogenate. The absorbed serum showed high specificity against human peripheral monocytes, suggesting the presence of similar antigenic determinants on J-III cells and human monocytes. Using this purified serum, an immunofluorescent technique was used to demonstrate macrophages in the site of pathology in various immunological diseases. In synovial tissues from patients with rheumatoid arthritis, positively stained cells were observed in the center and surrounding area of the germinal center. Small numbers of positive cells had also infiltrated around small vessels. Mesangial cells of renal biopsy specimens from patients with SLE were not stained. In Sjogren's disease, salivary glands showed granulomatous change, in which small numbers of diffusely scattered positive cells were observed. Similar changes were seen in Hashimoto thyroiditis. Patients with Behcet's disease showed only a few positive cells in the site of erythema nodosum. There were a small number of infiltrated positive cells in biopsied skin at the site of a positive tuberculin reaction.