Impaired immune regulation is considered to be involved in the pathogenesis of primary Sjögren's syndrome (primary SS). A qualitative T lymphocyte defect is suggested by previous reports of impaired lymphocyte proliferation, impairment of interleukin-2 (IL-2) production, and a decreased number of IL-2 receptors, whereas the capacity of monocytes to produce IL-1 is normal in most patients. We here report on the IL-1 sensitivity of monocyte-depleted mononuclear cells (MDC) from patients with primary SS. IL-1 responsiveness, evaluated by measuring the enhancing effect of an IL-1 standard on the proliferative response of the patient's MDC, was decreased compared to that of the control group, and there was a positive correlation between patients' IL-1 production and IL-1 sensitivity. The results confirm the notion of impaired T lymphocyte function in primary SS, and suggest a pathogenetic mechanism that involves both monocytes and T lymphocytes.