A sensitive microassay using sera from which endogenous bound insulin was removed by acid-stripping was employed to screen for insulin antibodies in 48 newly-diagnosed Type 1 diabetic children and 20 age-matched controls. Using intact diabetic sera, the binding of 125I-insulin exceeded the upper control range in 16/48 (33%); when the sera were acid-stripped at pH 4.2 for 30 minutes at 4 degrees C, binding to the 16 positive sera increased further and in another five sera became significant, making a total of 21/48 positive (44%). Children 5 years of age or less had a higher prevalence of insulin antibodies. Islet cell antibodies, assayed by indirect immunofluorescence, were detected in equal fractions of insulin antibody-positive and -negative sera (76% versus 77%). Assay of insulin antibodies using acid-stripped sera under the conditions described significantly increases the sensitivity of insulin antibody detection. Insulin antibodies appear to be a specific marker of islet autoimmunity, but compared to islet cell antibodies are relatively insensitive, and may be a marker for a particular subgroup of diabetics.