The synthesis of tyrosyl human C-peptide, a sequence of 32 amino acids, by the fragment condensation of the N-terminal octapeptide and C-terminal tetracosapeptide is described. The t-butyl protecting groups were removed by trifluoroacetic acid to obtain N-benzyloxycarbonyl-tyrosyl C-peptide. The hydrogenolytic debenzyl-oxycarbonylation of this derivative proceeded to an extent of only 80-90%, and tyrosyl C-peptide was purified by preparative electrophoresis. This purified tyrosyl C-peptide led to an improved sensitivity of the radioimmunoassay. The synthetic tyrosyl C-peptide in an immunoassay using anti human b-component serum reacted slightly differently from the synthetic human C-peptide. After labelling tyrosyl C-peptide with 125I and then purifying the radioactive product, we observed that 80% of the radioactivity could be bound when reacted with an excess of the serum. The circular dichroism spectrum of tyrosyl C-peptide is very similar to that of synthetic human C-peptide. An analysis of the spectrum indicates that 3-7 amino acids are in the beta-structure and the rest in random coil conformation.