The influence of type and degree of substitution of B 512 dextran fractions on reactivity with rabbit antibodies against unmodified B 512 dextran was studied in heterologous passive cutaneous anaphylaxis in guinea pigs. Low degrees of substitution, i.e. 1 substituent group per 7–30 glucose residues, affected reactivity with anti-dextran only slightly or not at all. This was shown for the following substituents: sulphate, carboxymethyl, phosphate, diethylaminoethyl, hydroxypropyl, butyryl, caprylyl, stearoyl, and fluoresceinoyl groups. With high degress of substitution, i.e. 1–3 substituents per 2 glucose residues, reactivity with anti-B-512-dextran is completely abolished for some substituents, and a new immunological identity conferred on the substituted dextran. Strongly charged groups like sulphate, carboxymethyl and diethylaminoethyl abolish reactivity with anti-B-512-dextran at relatively lower degrees of substitution than more neutral groups like methyl and acetyl. The anti-B-512-dextran represents a specific reagent for α-1,6-linked polyglucose, as evidenced by complete cross-reactivity with synthetic linear dextran; its specificity is emphasized by non-reactivity with α-1,6-linked synthetic mannan, the monomeric residues of the two polymers differing only in position of the C2 hydroxyl groups.