Human proinsulin (HPI) and C-peptide (HCP) were visualized by specific monoclonal antibodies (Mab's) in the conventional indirect immunofluorescent assay for ICA (islet cell cytoplasmic antibodies) on frozen sections of human pancreas. Two different Mab's GS-9A8 (anti-HPI, mouse IgG1) and GN-ID4 (anti-HPI/HCP, rat IgG2a) showed both intense islet cell cytoplasmic staining. In contrast to the anti-HPI staining which was confined to cell bodies only the GN-ID4 Mab produced an additional extensive staining of treadlike structures radiating from the islets. Both types of staining were blocked by excess HPI. Staining of fixed islet cell tissue were identical for the two different antibodies. Double-staining experiments clearly demonstrated that HCP but not HPI may diffuse from islets in sections of frozen human pancreas. It is concluded that antibodies cross-reacting with HCP do not define (pro) insulin containing cells.