A two wavelength immunofluorescence system was used to demonstrate islet cell surface antibodies (ICSA) from juvenile diabetics and complement on the same viable rat islet cell. When incubated with rat islet cells at 4 C and at 37 C, ICSA specific for B-cells proved capable of binding complement. In contrast, ICSA specific for both B and non-B cells were able to fix complement on all cell types at 4 C but only on B-cells at 37 C. This study provides further evidence for the B-cell specific cytotoxic effect of ICSA in the presence of complement at physiological temperature. In addition, the ultrastructural events leading to B-cell destruction after ICSA and complement binding were studied by transmission electron microscopy (TEM).