Amyloid fibrils were isolated from spleen and thyroid obtained at autopsy from one patient (S.K.O.) of Jewish origin with familial amyloidotic polyneuropathy. Gel filtration on Sephadex G100 after solubilization in 5 M guanidine HCl yielded three major components with 14,000, 9,000, and 5,000 mol wt, respectively. The two larger components shared antigenic determinants with human prealbumin. Amino acid analysis and amino terminal sequence studies revealed the 14,000-mol wt protein to be an intact prealbumin subunit. The 9,000-mol wt fragment obtained in highest yield encompassed the region from position 49-127 and the 5,000 mol wt fraction encompassed the amino terminal of prealbumin (position 1-48). An amino acid substitution (Gly/Thr) was detected at position 49, where enzymatic cleavage occurred. Thus, several prealbumin-derived fragments, predominantly the carboxyl end, constitute the amyloid fibrils in a heredofamilial amyloidosis syndrome of dominant inheritance.