1. 1. A small proportion (<0.5%) of rainbow trout immunoglobulins (Igs) binds to MemSep® staphylococcal protein-A (SpA) cartridges. SDS-PAGE analysis of this fraction under reducing conditions, and immunoblotting analysis with anti-heavy (H)-chain and anti-light (L)-chain monoclonal antibodies (mAbs), showed that, in addition to the characteristic H and L chains of rainbow trout Ig, two other peptides were also retained. 2. 2. The first peptide (P1) has a Mr of 65,000 and is recognized by rainbow trout anti-H-chain mAbs; the second (P2) has a Mr of 24,000. Fast protein liquid chromatography (FPLC) and SDS-PAGE analysis showed that the P1/P2 complex has a Mr of about 465,000, and that the P2 peptide (a) lacks disulphide bonds, (b) is present in molar excess with respect to P1 and (c) does not react with rainbow trout anti-L-chain mAbs. 3. 3. These results indicate that the P1/P2 complex is not an Ig variant. 4. 4. The presence of these peptides should be taken into account when analysing the molecular basis of the interaction of nonmammalian Igs with SpA, or when using SpA coupled to solid supports to purify fish Igs with a low SpA binding ratio. © 1993.