We have investigated β2-microglobulin (β2M) synthesis and release by blood leukocytes and isolated mononuclear phagocytes. Recent interest in β2M has developed since the discovery that this protein forms amyloid fibrils in patients undergoing long-term, chronic hemodialysis and that these patients have greatly elevated levels of monomeric β2M in their circulation. Since hemodialysis-related factors that increase β2M production are unknown, we evaluated β2M production by freshly prepared leukocytes and monocyte-derived macrophages under a variety of conditions. We utilized a novel enzyme-linked immunoabsorbant assay to quantitate β2M concentrations, and monitored interleukin-1 and β2M synthesis by immunoprecipitation. Incubation of leukocytes with Cuprophan or Hemophan does not increase β2M release. Adherence of macrophages onto polystyrene or Cuprophan membranes induces neither interleukin-1 nor β2M synthesis or release. In contrast, interaction of macrophages with lipopolysaccharide, γ-interferon, tumor necrosis factor, or interleukin-1 induces synthesis and release of β2M, indicating that β2M synthesis is increased during macrophage activation. Exposing leukocytes or macrophages to changes in osmotic or oncotic pressure induces a rapid release of β2M and interleukin-1 into the cellular medium. These results suggest that during hemodialysis, β2M production is more likely to result from endotoxin contamination, or osmotic and oncotic changes, rather than direct interaction of mononuclear phagocytes with Cupro