AbstractC‐peptide and insulin antigenic sites in rat pancreatic β cells were both labeled by the protein A and immunogold techniques. Gold bead density in the secretory granules was correlated with granule size distribution. Our results demonstrate that the distribution patterns of antigenic sites for C‐peptide (which also represents proinsulin sites) and insulin are similar when granule size distribution is correlated to the density of antigenic sites. It was found that the ratio of C‐peptide to insulin in granules is independent of granule size. Examination of frequency histograms by a Kolmogorov‐Smirnov analysis excluded the possibility that the antigenic sites of C‐peptide and insulin are distributed in the same manner. Mean gold bead density for insulin antigen (97 ± 24) was slightly higher than C‐peptide density (64 ± 18). The efficiency of labeling was found to be very low; less than 0.1% of the theoretically estimated antigenic site