Radiolabeled antibodies were perfused into fibrin clots and fibrinogen gels formed in vitro to assess the reactivity of selected epitopes. An antifibrinogen monoclonal antibody (MoAb) (antibody 1D4/xl-f), directed against an epitope in the A alpha-chain C-terminal region (A alpha 241-476), bound to 35% of the epitope in crosslinked fibrin clots and 37% of the same epitope in factor XIII-induced fibrinogen gel networks. A different MoAb (4-2/xl-f, anti gamma 392-406) bound to only 7% of the epitope in both fibrin and fibrinogen gels. As expected, an antifibrin MoAb (antibody T2G1, antiB beta 15-21) did not bind to fibrinogen gels, but bound to fibrin, although to only 14% of the available T2G1-reactive epitopes. An antibody that does not recognize fibrin (antibody 1-8C6, antiB beta 1-21) predictably did not bind to fibrin clots and bound to 35% of the 1-8C6 epitopes present in fibrinogen gels, a level of binding also observed with antibody T2G1 and fibrinogen gels only after the latter were treated with thrombin. T2G1 epitope expression was affected much more than 1D4/xl-f epitope expression in clots formed in buffers of high or low ionic strength, conditions known to influence clot structure. Studies on the availability, in quantitative terms, of the T2G1-reactive epitope in fibrin clots is of particular importance because this antibody is currently being used in clinical trials as a clot imaging agent.