A process for the preparation of insulin precursors of the formula (I) ##STR1## in which R is hydrogen or the presequence P.sub.m --Q.sub.n --, in which P is a sequence of naturally occurring amino acids with m being from 0 to 50, Q are basic natural amino acids, and n is an integer from 1 to 4, Y represents --Lys.sup.B29 --Z.sup.B30 --, in which Z denotes Ala, Thr or Ser, and the bridge extending from A-1 to A-21 is an insulin A-chain, the bridge extending from B-1 to B-30 represents an insulin B-chain, and X is a bridge which is bonded to the insulin A-chain at the amino group of A-1 and is bonded to the insulin B-chain at the .epsilon.-amino group of B-29-- in which case it is bonded to the free bond of Z.sup.B30 OH-- or at the carboxyl group of B-30, from reaction mixtures which result from folding of insulin precursors from S-sulfonates of the formula (II) ##STR2## in which R, X and Y have the abovementioned meanings, which comprises the false recombinants contained in the reaction mixture being precipitated by adjustment of the reaction mixture to pH 4 to 6, the precipitate being removed in customary manner and converted by sulfitolysis into the S-sulfonate of the formula (II), and the latter then being subjected to renewed folding.