Chemotaxis of peripheral blood monocytes obtained from patients with Behcet's disease was examined under agarose, since only lymphocytes and macrophages were observed in biopsied specimens at the early stage of erythema nodosum and oral aphthous ulceration. Peripheral blood monocytes were used as the chemotactic cells and AB serum activated by zymosan was used as the chemotactic factor. The mean chemotactic distance of the monocytes in Behcet's patients was 86.9±11.6, as compared to 74.6±11.7 in normal controls. These results showed that the chemotactic activity in Behcet's disease was significantly higher than that of normal controls (p<0.05). Random migration was also significantly increased in Behcet's disease (p<0.05). The value of random migration was 47.1±13.8 in the patients and 35.7±7.9 in the normal controls. These data seemed to account for the mononuclear cell infiltration observed at the site of lesions in Behcet's disease.