A protein, ASCA, is isolated from amyloid fibrils extracted from heart tissue of five different patients with senile cardiac amyloidosis (SCA). The proteins of all five patients showed immunological identity when reacted with an antiserum raised against one of the proteins. In contrast, no reaction was obtained with antisera against a variety of other amyloid proteins. The antiserum against the subunit protein of senile cardiac amyloid did not react with any other amyloid preparations tested, nor with extracts of normal heart tissue. Thus, the subunit protein appeared to be unique to senile heart amyloid. The protein could form fibrils in vitro, had a mol wt of about 6,000 daltons and the amino acid compositions investigated in two cases showed extensive similarities but were clearly different from that of protein AA of secondary amyloid fibrils.