To investigate the involvement of proinsulin (one of the contaminant proteins of therapeutic insulin) in systemic insulin allergy, we studied seven diabetic patients who presented with generalized insulin allergy. Commercial and highly purified (single component) insulins produced almost identical responses on the intradermal tests. In all cases, [125I]bovine insulin and [125I]bovine proinsulin bound to the circulating reaginic immunoglobulins. The binding of [125A]proinsulin was blocked completely by unlabeled insulin and was uneffected by bovine C-peptide, indicating a cross-reaction of proinsulin with antinsulin reaginic immunoglobulins and the absence of proinsulin-specific reaginic antibodies. These observations suggest insulin, but not proinsulin, as the mediator of the immediate insulin allergic reaction.