A new latex particle agglutination test (FIBROTEX) has been compared with the Thrombo-Wellcotest and the tanned red cell hemagglutination inhibition immunoassay (TRCHII) for sensitivity to purified fibrinogen and plasmic degradation products and for reactivity with 31 normal sera and 170 sera from patients with thrombotic disorders. The latex particles in the FIBROTEX are coated with antibodies against fibrinogen and fragment E (anti-FE), in contrast with the anti-fragment D and anti-fragment E antibodies which coat the latex particles of the Thrombo-Wellcotest (anti-DE). The relative sensitivities of the TRCHII and the anti-FE and anti-DE latex tests were, respectively, 0.5, 2.5 and 7.5 microgram/ml for fibrinogen; 0.5, 0.5 and 4.0 microgram/ml for fragment X; and 1.25, greater than 30 and 8.0 microgram/ml for fragment D. Compared with anti-DE, the anti-FE particles demonstrated fewer false-positive results with sera and urines of normals and higher titers, fewer false negatives and better overall correlation with the TRCHII results (r = 0.89 vs. 0.75) on pathologic sera. The FIBROTEX is simple and rapid (5 minutes) and provides a more sensitive and accurate determination of serum fibrin degradation products than the Thrombo-Wellcotest.