The structural features of early fibrin oligomers produced during the initial stages of polymerization were investigated by rotatory shadowing after cryotechnical preparation. The building blocks of polymerization, namely fibrin monomer units (in analogy to fibrinogen itself), were found to exhibit a high degree of flexibility which is independent of fibrinopeptide A and B removal. Early polymers exhibited directed longitudinal growth and were frequently branched. Along the main oligomer axis, fibrin monomer units were randomly orientated. Within early oligomers, a given fibrin monomer unit was found to establish a single contact with each of its two neighbors, suggesting that during the early stages of polymerization, only one polymerization and one binding site are activated per fibrinogen molecule (becoming an AB2 fibrin monomer unit). This morphological feature was corroborated by the finding that early oligomer fractions are deficient in only 50% of releasable fibrinopeptide A. Early associations between AB2 fibrin monomer units were demonstrated to be reversible and to occur in the absence of direct domainal contact; interactions thus presumably occur via fine molecular protrusions on either D or E domains. The arrangement of AB2 fibrin monomer units within early oligomers suggests that, with respect to their structural organization, fibrinogen molecules are radially symmetrical through the E domain (implying an antiparallel organization of polymerization and binding sites). This pattern is inconsistent with a "top-bottom" model, and thus with "half-staggered double-stranded" polymer growth. The methodological problems responsible for the apparent conflict with previous morphological findings are discussed.