Antisera were raised against degraded amyloid fibrils isolated from the heart of a patient with senile cardiac amyloidosis (SCA), and from a medullary carcinoma of the thyroid (MCT). The antisera were absorbed and used in indirect immunofluorescence to identify an amyloid fibril protein (ASCA) in heart tissue from patients with senile cardiac amyloidosis and to identify the amyloid fibril protein (AMCT) found in association with medullary carcinomas of the thyroid. Absorbed anti-ASCA antiserum did not react with normal tissue such as heart, liver, spleen, and striated muscle, or with amyloid tissue known to contain amyloid fibril proteins AA, AλI, AλIV, AλV, AMCT or with pancreatic tissue containing islet amyloid deposits. The reactions with senile amyloid heart tissue could be blocked completely by degraded amyloid fibrils extracted from senile amyloid heart tissue or by amyloid fibril protein ASCA isolated from such fibrils. The anti-AMCT antiserum showed a similar specific reaction restricted to amyloid associated with MCT. In addition, antisera specific for amyloid fibril proteins AA, AλI, AλIV, and AλV failed to react with senile cardiac amyloid, pancreatic islet amyloid, or medullary thyroid amyloid.