A scanning transmission electron microscope (STEM) was used to examine ultrathin sections of rabbit white skeletal muscle. Lactic dehydrogenase (LDH) activity was localized in the tissue using the tetra-nitro blue tetrazolium (TNBT) method. For most specimens postfixation was omitted in order to avoid reoxidation and solubilization of the formazan by osmium tetroxide. The STEM image revealed sufficient contrast of the intracellular structures and apparently electron-dense reaction product in the sarcoplasmic reticulum and mitochondria. Substantially less contrast was obtained when the same areas were observed by conventional transmission electron microscopy (CTEM). In material postfixed with osmium tetroxide, although the tissue contrast was improved, the TNBT reaction product was focally leached out, exhibiting lower contrast than in unosmicated sections. These results indicate that the fine structural visualization of dehydrogenases with TNBT, the STEM technique as used in the present study is superior to that obtained by CTEM.