A radioimmunoassay capable of measuring 1 pmole of human fibrinopeptide A has been developed, and should prove useful to detect the release of this peptide from fibrinogen during the coagulation process. Antibodies to fibrinopeptide A were produced by injecting New Zealand white rabbits with a mixture of Freund's adjuvant and native fibrinopeptide coupled to human albumin. N-Tyrosyl fibrinopeptide A was synthesized by the solid-phase method, and was iodinated with (125)I by the Chloramine-T method. 48-73% of the radiolabeled peptide could be bound by the serum of a rabbit immunized with the fibrinopeptide-albumin preparation. Antibody-bound peptide was precipitated by dioxane and was thus separated from unbound peptide. The addition of excess native fibrinopeptide to the radiolabeled material prevented its binding to serum. Native fibrinopeptide A and synthetic fibrinopeptide A were identical in their ability to prevent binding, whereas fibrinogen was from 1/25,000 to 1/50,000 as effective on a weight basis. Plasma filtered through a membrane relatively impermeable to molecules larger than a molecular weight of 34,000 showed no fibrinopeptide reactivity, whereas a similar filtrate of serum gave quantitative recovery of fibrinopeptide reactivity.