Aspects of lymphocyte function were investigated, in vitro, in nine patients with active Behçet’s disease. The rosette formation technique revealed a slight decrease in number of T lymphocytes and more so of B lymphocytes. In spite of their low number, the transformation of the T lymphocytes in response to mitogens did not show a significant difference from that of normal controls. This may indicate a hyperreactivity, and in two instances a spontaneous blastogenesis prior to addition of mitogens, was also noted. The immunoglobulins and complement levels in serum were within normal limi