Spleen cells from acutely diabetic (AD) and non-diabetic but diabetes prone (DP) BB/Wor rats lysed insulinoma target cells to a significantly greater degree than did diabetes resistant (DR) cells as determined using a 51Cr release cytotoxicity assay. There were no differences between the AD and DP groups. Lysis was not target cell specific, since somatostatin secreting RIN 14B cells, Wistar Furth leukemia cells designated LW12, PC12 cells and NK sensitive YAC-1 cells were also lysed. Lysis of all target cells was significantly reduced by pretreatment of the effector lymphocytes with antiserum to NK cells (anti-asialo GM1) and complement suggesting that NK cells mediated destruction of these cells. These data demonstrate a generalized increase in non-specific NK cell activity in BB/Wor rats. Since NK cells have been shown to mediate antibody dependent cell mediated cytotoxicity (ADCC), splenic lymphoid cells from AD rats were tested for their ability to lyse insulinoma target cells in the presence of diabetic rat sera which were demonstrated to contain islet cell surface antibodies. Three different ADCC protocols were tested but in each case the addition of serum dilutions from AD rats reduced the lysis of insulinoma cells by AD spleen cells in a dose dependent manner. This inhibition was also demonstrated when sera and effector cells from control rats were used. As a positive control, DR spleen cells were incubated with 51Cr labelled target cells that were untreated or pre-treated with anti-rat class 1 antibody (OX18). Pre-treatment of the target cells resulted in a marked increase in their subsequent lysis.(ABSTRACT TRUNCATED AT 250 WORDS)