Neutrophils of 31 patients with neutropenia and rheumatoid arthritis (RA) have been studied to assess their ability to generate superoxide radicals (O-2) on activation. Seventeen patients had classical Felty's syndrome and 14 presumed chrysotherapy related neutropenia. Results were compared with those from age and sex matched controls with uncomplicated RA and from normal subjects. Neutrophils from patients with Felty's syndrome had a significantly reduced ability to generate superoxide radicals when compared with the other three groups. In addition, serum levels of IgG polymorphonuclear leucocyte binding activity (IgG PBA) were also raised in the group with Felty's syndrome. A statistically significant inverse correlation existed between O-2 generation and IgG PBA. It is concluded that neutrophil reactive IgG may have an important role in both quantitative and qualitative defects in neutrophil function in Felty's syndrome.