A fraction of synthetic linear dextran (D-8) of Mn 26,900 was tested for cross-reactivity with rabbit anti-B512 dextran antiserum (RAD) and compared to a B512 dextran fraction (D-40) of similar molecular weight distribution. B512 dextran is a linear α-1–6 linked polyglucose with few side chains: about one branch per 20 glucose units; at least 80% of branches comprise only 1–2 glucose residues. The synthetic dextran is a completely linear α-1–6 linked polyglucose. In Ouchterlony gel double-diffusion, both D-8 and D-40 produced single precipitate lines; in a comparative array, complete fusion of precipitate lines of D-8 and D-40 indicated the identity of antigenic determinants. In quantitative precipitation and single radial immunodiffusion, D-8 and D-40 precipitated comparable amounts of antidextrans and produced precipitate haloes of similar size. Systemic lethal anaphylaxis could be elicited by D-8 in passively sensitized guinea pigs, and protection from lethal shock was obtained by adding isomaltooctaose to D-8 upon challenge. Similarly, passive cutaneous anaphylaxis (PCA) of comparable intensity was elicited by D-8 and D-40 upon challenge of sensitized guinea pigs. Neutralization of PCA by adding D-8 to RAD prior to intradermal sensitization and challenge with D-40 also indicated the identity of antigenic determinants. These findings are taken as evidence that D-8 is a homodeterminant antigen with many determinants, and that the antigenic determinants of both linear synthetic and B512 dextran are repetitively arranged along the molecular chain.