The recently developed technique of reductive animation, followed by gold labeling, was applied to visualize the reducing ends of cellulose microcrystals from cellulose I, cellulose II, and cellulose IIII. In these crystals, which were also characterized by electron diffraction, the labeling proved that the chains were organized in a parallel fashion in cellulose I from ran-tie and Valonia and also in cellulose IIII from Valonia. In microcrystals of cellulose II from mercerized ramie, the labeling method showed that the chains were packed into an antiparallel mode. These results are discussed in terms of the fine structure of cellulose I where neighboring microfibrils of opposite polarity are visualized. The mercerization process whereby cellulose I is converted into cellulose II is therefore best described in terms of an intermingling of the cellulose chains from neighboring microfibrils of opposite polarity. As opposed to the case of mercerization the conversion of cellulose I into cellulose IIII does not require the participation of neighboring microfibrils since the crystalline domains are converted individually.