A method is described for the synthesis of high-molecular-weight carrier ampholytes for preparative isoelectric focusing of peptides. A giant polyethylene imine (Mr 40 000-60 000) is mixed with a linear gradient of acrylic acid in a flow-through system and let to react at 80 degrees C for 70 h. Giant carrier ampholytes (Mr range 50 000-90 000) are thus obtained. These compounds interact very strongly among themselves, probably not by hydrogen bonds or hydrophobic interactions but by ionic bonds. In fact, the aggregates are split by high salt (NaCl) or by zwitterionic compounds (Gly, taurine) or at acidic or alkaline pHs. They appear to interact only weakly and reversibly with proteins and no interactions are apparent with model dipeptides (His-Ser, His-Met, His-Phe and His-Lys).