An unexpected cross-reactivity between trout immunoglobulin (Ig) and keyhole limpet hemocyanin (KLH) was observed. Rabbit antisera to KLH were capable of binding to radioiodinated trout Ig and, conversely, antitrout Ig reacted with KLH. The cross-reactive antibodies were not found in preimmune sera and did not arise because of a common contaminant in the two immunizing preparations. The molecular basis of the cross-reactivity was found to reside in the carbohydrate moieties. Isolated glycopeptides from KLH and trout Ig were efficient inhibitors of the croos-reactivity. Furthermore, l-fucose was capable of inhibiting the cross-reactivity, whereas other monosaccharides tested did not. Absorption of anti-KLH with trout Ig and anti-trout Ig with KLH effectively removed the cross-reactive antibodies and only slightly affected the titer to their respective homologous antigens. Antibodies with specificity for l-fucose were isolated from anti-KLH and anti-trout Ig sera by passage over affinity columns and elution with the monosaccharide.

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