The relationship between immune insulitis and glucose tolerance was investigated in three groups of mice following active immunization with different components of bovine pancreatic hormone. An abnormal blood glucose level was observed in the three groups ranging from 33.3% to 87.5% of sensitized mice. A relationship was not present between the glucose tolerance response and the presence of insulitis or anti-insulin antibody in the blood of sensitized mice. However, all sensitized mice with a marked decrease in glucose tolerance were found to have insulitis. In animals without established insulitis and with no demonstrable anti-insulin antibody, abnormal glucose tolerance was noted. This latter condition occurred more frequently with recrystallized insulin than with a-component and did not occur with monocomponent insulin. These findings seemed to indicate that two distinct processes involving some circulating antibodies with anti-insulin antibody and insulitis might be involved in the development of the observed glucose tolerance abnormality.</p