The effects of the ligands phenol and resorcinol on the crystallization of human insulin have been investigated as a function of pH. Powder diffraction data were used to characterize several distinct polymorphic forms. A previously unknown polymorph with monoclinic symmetry (P2(1)) was identified for both types of ligand with similar characteristics [the unit-cell parameters for the insulin-resorcinol complex were a = 114.0228 (8), b = 335.43 (3), c = 49.211 (6) Å, β = 101.531 (8)°].