“Single-peak,” “single-component,” and “monocomponent” insulins have been produced in an attempt to eliminate insulin antigenicity. Recently “single-peak insulin” has been shown to be antigenic. From animal experiments and preliminary human studies it has been claimed that monocomponent (MC) insulin is nonantigenic or only negligibly so. In this study the antigenicity of MC insulin was determined in two groups of diabetic patients. In group 1, seven patients treated with insulin for the first time were given MC insulin for seven to fifteen months. Four of the seven patients developed significant IgG insulin antibodies after four to ten months. In one patient the IgG insulin antibody concentration was high (8.51 mU./ml.). In two patients IgG proinsulin-specific antibodies were detected. In group 2, fourteen patients with unstable diabetes, insulin allergy, or resistance were changed from conventional to MC insulin. Treatment with MC insulin did not decrease insulin requirement or improve diabetic control when assayed by the M factor. After seven to eleven months of therapy there was no significant fall in insulin antibodies except in two patients in whom corticosteroids had been administered simultaneously. These results differ significantly from those previously reported and could be interpreted as suggesting that insulin itself is antigenic. When the purity of the MC insulin was determined, significant contaminants could be demonstrated in all of ten separate batches of MC insulin. Gel chromatography, polyacrylamide gel electrophoresis, and proinsulin radioimmunoassay were used to identify the presence of nonconvertible insulin dimer, proinsulin, and monodesamido insulin in antigenically significant concentrations. The generation of IgG insulin antibodies in MC-insulin-treated patients cannot be interpreted as a true indication that insulin itself is antigenic. The problem of insulin antigenicity has not been resolved and will not be until a highly purified insulin is available. Unfortunately, the MC insulins do not meet these requirements.