The present invention comprises a process for preparing insulin or an insulin derivative with correctly linked cysteine bridges from a precursor of said insulin or insulin derivative, wherein said precursor is subjected to a folding process in the presence of cysteine or cysteine hydrochloride and a chaotropic auxiliary compound. The insulin or insulin derivative with correctly linked cysteine bridges is obtained by enzymic cleavage by means of trypsin or a trypsin-like enzyme and, where appropriate, additionally by means of carboxypeptidase B and subsequent purification on an adsorber resin, which process is carried out at varied pH and temperature ranges.