The usefulness of the measurement of serum antibody-bound and total immunoreactive insulin (IRI) concentrations in the assessment of insulin antibodies was evaluated in a material comprising 49 insulin-dependent diabetic children with a mean age at onset of 8.6 years (range 0.8-16 years) treated with highly purified porcine insulins. Serum antibody-bound and total IRI concentrations of individual patients were compared with insulin antibody levels measured with 3 different insulin antibody assays. The correlation coefficients of insulin antibody levels with concentrations of serum antibody-bound IRI ranged from 0.75-0.79. In serum samples with moderate or high insulin antibody levels most of the insulin was in the form of insulin-insulin antibody immunocomplexes. Thereby a very close correlation was found between antibody-bound and total serum IRI concentrations (r = 0.98) in this material. Residual endogenous insulin secretion decreased with increasing duration of diabetes. No significant correlation was found between the duration of diabetes and serum antibody-bound IRI concentrations. High serum antibody-bound IRI concentrations were associated with low glucagon-stimulated plasma C-peptide levels. Although the determination of serum antibody-bound IRI concentrations does not characterize insulin antibodies with regard to binding capacity and affinity constants, it yields information of the actual degree of insulin binding in the circulation. This information may be useful in assessing the benefits of transferring diabetics with high insulin antibody titers from conventional to highly purified porcine or human insulin therapy.