Standardization of the measurement of amyloid degrading activity (ADA) by diffusion of serum in amyloid-impregnated agar plates may either indicate or exclude with reasonable certainty the presence of systemic AA amyloidosis. In certain cases, it may obviate the need for a diagnostic biopsy. The sera of 38 patients with systemic amyloidosis were tested and compared with sera of 38 controls matched for age, serum creatinine and albumin blood levels, and with sera of 48 additional controls with the same basic diseases as the amyloidotic patients but without amyloidosis. The difference between ADA of amyloidotic and control patients was significant, with no overlap in the range of activity between the two groups. A positive correlation was found between ADA and serum albumin concentration in the nonamyloidotic matched controls but not in patients with amyloidosis. Our data do not support the view that the decline in ADA of sera of amyloidotic patients is due to hypoalbuminemia.