We determined the effects of plasmapheresis on cytotoxic antibodies to islet cells in 10 children (aged 11–16 yr) with newly diagnosed insulin-dependent diabetes mellitus (IDDM), as well as the plasma levels of antibodies over the next 30 mo and their relation to serum C-peptide concentrations. Complement-dependent, antibody-mediated cytotoxicity (C′AMC) in plasma was measured in a 51Cr release assay using monolayers of the rat islet cell line RINm5F. Cytotoxic antibodies decreased in most IDDM subjects treated by plasmapheresis four times within 2 wk of diagnosis; however, the decreases were small and lasted less than 2–3 days. Thus, both before and after plasmapheresis, 7 of the 10 IDDM children were C′AMC-positive (51Cr release > 2 SD above mean for 13 healthy children). After 18–30 mo, only 2 of the original 7 IDDM children with C′AMC-positive plasmas were still positive, and 1 of the original 3 IDDM children without significant cytotoxicity had become positive. Meal-stimulated serum C-peptide responses, measured from diagnosis to 18–30 mo later, did not correlate with C′AMC values. We conclude that (1) plasmapheresis has only transient effects on islet-cytotoxic antibody levels in children with IDDM; (2) these antibodies decrease in most, but not all, subjects over the first 18 mo after diagnosis; and (3) the level of cytotoxic antibodies in IDDM plasma at diagnosis has no predictive effect on residual B-cell function.