Antisera have been prepared against the major nonimmunoglobulin component of secondary and familial Mediterranean fever (FMF) associated amyloid which has been called A component or acid soluble fraction (ASF). The antisera were shown to be monospecific for ASF by precipitation of 125I-labeled antigen and gave a reaction of identity with four different ASF preparations. The antisera were able to detect a circulating component in human serum that migrated in the α1-globulin region. This circulating component gave a line of identity with degraded ASF by double immunodiffusion. 57 normal sera and 89 sera from patients with diseases known to be frequently associated with amyloidosis were tested by immunodiffusion for the circulating ASF component. 7% of normal sera and 50–80% of the pathologic sera had elevated amounts of this component. Absorption studies showed that all normal sera probably have small amounts of this component while cord sera do not have detectable amounts. This component was partially purified and was shown to be slightly larger than albumin. The relation of the circulating component to the acid soluble fraction of amyloid is discussed.

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