Abstract Anti-fibrinopeptide A antisera with different immunochemical specificities for human fibrinopeptide A (FPA), when it is part of fibrinogen or fibrinogen fragments, were used in the radioimmunoassay of FPA to demonstrate different susceptibility of native and purified molecules to degradation. Unprocessed human plasma fibrinogen had an FPA immunoreactivity with one antiserum, K2, equivalent to 0.02% of the calculated total amount of FPA in the fibrinogen, while different batches of purified fibrinogen crossreacted equivalent to 0.4–8.7% of their calculated total content of FPA in the fibrinogen. The cross-reactivity of the different batches was correlated with their dialyzable FPA immunoreactivities and was increased with storage and longer incubation times, as was also the cross-reactivity of FPA-containing fibrinogen fragments.