The potentiating effect of the soluble factors released from normal or diseased lymphocytes on neutrophil functions were investigated in the presence or absence of mitogens and wall preparations of Streptococcus pyogenes. When normal T lymphocyte populations were stimulated with T cell mitogens or with streptococcal preparations, the supernatants from these cultures potentiated neutrophil chemotaxis, phagocytosis and O2- generation. Upon gel-filtration of these stimulated lymphocyte supernatants, the neutrophil-potentiating activity was inactivated by trypsin or by a 30-min incubation at 130 degrees C, but was not affected by acid treatment at pH 2 or heat treatment at 56 degrees C for 60 min. Its activity was almost not affected by antisera against human interleukin-1, interleukin-2, interferon-gamma or tumour necrosis factor. With the stimulation of T cell mitogens, the supernatants released from the lymphocytes of not only the patients with Behçet's disease but also healthy and diseased controls enhanced neutrophil functions. However, supernatants from streptococcal preparation-stimulated lymphocytes from patients with Behçet's disease had a higher potentiating effect on neutrophil functions. Our study suggests that the enhanced neutrophil functions in patients with Behçet's disease may be related to an abnormally high level of circulating activated T cells in these patients.