An electron microscopic method was applied to the quantitative analysis of silver clusters formed on fine cubic AgBr grains by light and reduction. The observation of only one light cluster, i.e., one latent image center, per grain and many reduction clusters per grain indicated a difference in the mechanism of cluster formation, i.e., the presence and absence of the concentration principle, respectively. We concluded that the quantum yield for the photochemical formation of clusters could be unity if positive holes were prevented from reacting with growing clusters. This was consistent with reported quantum yields for print-out silver on the basis of chemical analysis, and therefore proved the applicability of the present method for the quantitative analysis. It was also found that the formation of reduction clusters on treatment of AgBr grains with DMAB occurred nearly quantitatively. The analysis of size distribution of reduction clusters gave additional support for the proposal that DMAB formed dimers of silver atoms, which were stabilized to act as reduction sensitization centers at surface kink sites, but which aggregated to form large clusters when their amount was larger than that for the saturation of the kink sites.