This study was undertaken to determine humoral immune response to the presence of anti-immunoglobulin antibodies in children with newly diagnosed type 1 diabetes mellitus, using as a target cow immunoglobulins, in an attempt to elucidate further complex immuno-pathogenetic interactions of the disease. Serum immunoglobulin G (IgG) concentrations were measured by ELISA in 30 children with type 1 diabetes mellitus and 30 healthy matched normal children. It was found that normal children had a mean IgG level of 7.41 mg/ml while diabetic individuals had a mean IgG level of 8.52 mg/ml (p<0.00004). On the contrary, the mean level of IgG in diabetic sera after purification from anti-cow immunoglobulins was determined to be 7.52 mg/ml. Therefore, there was no significant difference in IgG level in patients with type 1 diabetes mellitus after removal of anti-cow immunoglobulin antibodies compared to normal children (p<0.58). Visualization of IgG and immuno-precipitation confirm that anti-cow immunoglobulins antibodies, which were unrelated to antigen, were co-precipitated with the antigen-antibody complex. A circulating immunoglobulin reacting with other immunoglobulins is thus present in children with type 1 diabetes and may well play a part in the complex immuno-pathogenetic interactions.