Antigen expression is studied corresponding to a monoclonal autoantibody (IC2) derived from a hybridoma of rat myeloma Y3 cells and splenocytes of the diabetic BB rat. The selective reactivity of IC2 with islet cells has earlier been proven. We studied the possible specificity for beta islet cells, and the possible variation in autoantigen expression. Islet cells were isolated by cautious collagenase and dispase treatment. The cells were labelled with IC2 alone or together with anti-insulin immunoglobulin in double-labelling experiments. Extensive series of cells were examined by immunofluorescence microscopy, and some samples also by flow cytometry. In double-labelling examinations we found that only anti-insulin positive cells could bind the IC2 antibody, thus showing beta-cell selectivity. On the other hand, not all anti-insulin positive cells were IC2-positive. Since insulin treatment has been shown to decrease the incidence of diabetes in the BB rat, islet cells were examined after reduced beta-cell strain. Islet cells from Lewis and Wistar Furth rats display 21.4 +/- 1.4% IC2-positive cells, while islet cells from 24-hour fasting animals showed 7.0 +/- 1.4% (p less than 0.0001). Similar results were seen for BALB/c mice (25.0 +/- 1.8% vs. 13.7 +/- 2.3%, p less than 0.002). Also, after a week of insulin treatment, autoantigen expression was significantly decreased. Thus, the IC2 antibody is beta-cell-specific, and expression of the corresponding cell surface antigen depends on the functional state of the beta-cells.